Aiptasia is a pest
anemone that can take over your tank if you don't get rid of it. They can be brown, yellow, clear, or a few other colors. If you aren't sure if something is aiptasia, poke it with something. If it sucks into the rock, it is likely aiptasia. If it just folds up, it is probably a harmless polyp,, ways to rid are buying joes juice or aistasiax
or injecting with lemon juice or if you cant get rid of a last resort is pour boiling water over the rock but this will kill anything living in the rock but can be reseeded with more live rock