Very bad... Release spores when torn or ripped which become new aiptasia, leave "buds" of new aiptasia in its path, and can sting corals to death. There are a few ways to treat which aiptasiaX is one of the best methods but before you go blasting it with aiptasiaX or lemon juice let's get a positive ID because thru the haze it is very hard to tell, almost just a silhouette.