If I had some once live rock that when it was alive had algae on it but it has been dry for over a year would it be safe to put it in the tank now or is there something I should do first to make sure that they algae and other stuff doesn't come back?
Power washer if you need it. dip it till it is clean, then cure it outside (perfect time right now!) .
cover it for no light.
basically you are cooking/curing or whatever you want to call it. It takes longer because there is no bacteria on it, like when it comes from the ocean as "live rock". If you can put some live rock in there it helps the process go faster. with good waterchanges and o skimming you could be done in September.
You know it is going well when stuff stops dropping out of the rocks.