This popped up a couple weeks ago in my son’s softy tank, is this a baby hairy mustoom, gsp? Never seen one of these and we are mystified haha.
Looks like a ball anemone to me. I had them as hitch hikers and they never caused any problems. Some people have had them become a problem but usually not on the order of aiptasia. If you want to get rid of it you could use Aiptasia X . I think they're pretty.
Doc is correct 100% Ball Anemone. They are harmless but some people hate them. If you have one you probably have more. They will be located in dark or low lit, low flow areas.