Your not at a eutrophic state. But algae's natural processes promote a increasing nutrient levels. Which your maintence and water changes ect are keeping it from progressing rapidly. Your not operating at low nutrients or ultra low (oliotrophic). Your tank is hovering probably mesotrophic or at the edge of low nutrients.
Corals can survive and some do really well at that range. Softies tend to love it. Even some Lps. Sps can tolerate it but prefer lower. Due to the growth rate being decent places you higher then you think. Low or ultra low algae does not gain biomass/growth. It instead starves and diminishes.
Algae can act as a pseudo buffer of sorts, but it's releasing nutrients when you start to go towards lower nutrients. But slows the climb to eutrophic. The tank problems arrise when the increase bioload out paces your maintence and rocks ect start loading phosphates. This happens easier with algae as bacteria are being fed fro more algae sources as opposed to removing bound phosphates. They go for the easier meal.
This is where you have to step back and look at what the algae is effecting. Algae's a source of direct and released organic material. This is prime food for bacteria and pods. So its in essence a buffet for organisms. So since their is a easy access buffet you have far less working on cleaving and removing bound phosphates from the rock/coral skeletons in the system. This imbalance causes a build up of bound phosphates which once they reach high enough levels algae and other nuisances can take root on "fertile" fields of sorts. They now have a sponge of phosphates in high concentration to move form just the ATS to the rock work as well. This is one way algae slowly creeps up the nutrient level in a system.
Now keep in mind more bacteria is not a good thing really. The more bacteria you have in a system processing waste means there is more nutrients trapped within all those life forms. So the basic Bioload of a system goes up greatly. While day to day operation this is never noticed, but suspended in the tank is a much larger stock pile of nutrients. While its static while populations remain stable, a disruption to the tank. Whether be it be dosing to much of something, or lights weakening or changing, even just a air borne house chemical killing off some, any one of the human errors our tanks experience. Well due to having this large stock pile of nutrients waiting for the first misstep you run higher risks with a crash of sorts. One big enough misstep and all the bacteria population has die off or starves from a extra good cleaning. Those dead bacteria are now releasing nutrients not containing them. So you get a rise in nutrients again. Algae and bacteria play a dance one fuels the other, this ends up stacking up nutrients and in essence is a army of nutrients waiting to be released.
In the end your tank sounds like your cleaning is good enough that you might be keeping things in check for now. The algae is working for you at times generally but also working against you at times feeding nutrients back. It is preventing you from really pushing the nutrient levels ultra low via maintenance, but it can slow a sudden increase. Granted with some risk of loading phosphates in the rock and causing a later issue. Rock loading is not a sure thing, as your cleaning routine can prevent it or slow it, or if its not enough it just buys you time. This is why the presence of algae growth is not a sign of a system reducing nutrients. Algae cannot reduce nutrients past a certain point, once it hits that point it works against you.
I have not problems with algae, beyond the idea of in a Reef Tank it allows you to lower the nutrient levels to low nutrients or Ultra low. Because at that point its actually fighting you and you are making the system lower with algae kicking and screaming the whole time until it dies.
My low nutrient tank, has very minimal water changes, and a pretty massive food importing daily. I run nothing but a skimmer as well. As the tank has matured I have starved out all the algae down to very little trace. I even watched some byropsis decay and die. (Some types of Byropsis have the lowest nutrient threshold before full decay of algae). I have run a screen ATS on this tank a little while back to see if I could even grow algae on the screen. I couldn't grow any more them film algae on it. Even still if you noticed I said its a "low" nutrient tank. I am nearing Oliotrophic but I still have a ways to go. Simply I have some purging of bound phosphates that once they have purged I should firmly be able to hit ultra low levels. My 150g has been a fun test bed for nutrient control, and I love to watch algae starve it makes me feel good knowing I am going in the right direction.
I'm sadistic with algae, I let my 30g go with not water changes for a month to allow the one bubble algae to grow some friends in the 30g. I then took the bubble algae and popped half of them in the 150g and placed the remaining ones into a crack with light to see if they would survive. Yes I am twisted to invite bubble algae to come play. But when it died and no new ones grew, it was a bio indicator that the tank was moving the right direction.
Its not done purging even now. I can still see the bio indicators of continual reduction of nutrients. I figured it would take at least a year to get it in the realm of where I want it to be. It will be a bit longer.
