Do you have a picture of it? "grassy growth" sounds like Hair Algae. If its Hair Algae, a lawnmower blenny or a tang (depending on your tank size) will eat it. But, you need to figure out where the source of the HA's food (phosphate) is coming from and solve that problem and the HA will go away without a food source.
The rocks that do not have anything else on them, take out and scrub with a stiff bristled toothbrush and get as much as you can off. Do the rocks with corals but you will need to put them in a bucket of water so you don't keep them in air too long. After you physically get as much as you can off, do a major water change. With it just being a 24, do a 10-12 waterchange. Then wait a week and repeat it. You can get rid of it, but its going to take some work.