yup aiptasia - harmful pest that can spread if left unchecked.
do not try to manually remove them, scrape them off, or cover them with puddy/crazy glue b/c if they sense harm they will retract and spread their spawns into the water column and you'll end up with them popping up everywhere.
There are multiple effective ways to control them - Aiptasia X, F-Aiptasia, lemon-juice, peppermint shrimp, nudys.
most of us will have it in our tank at one point or another. IME once you have it, it'll be very difficult to completely eradicate them, but any of the above are very effective at controlling them if done correctly. I personally used F-aiptasia. Used to have a 30+ outbreak that i fixed over a 2 week period. now i have maybe 1 popping up every few weeks.
