I have those in my tank as well someone told me that they are called strawberry nems. They haven't harmed anything yet but there is defiantly more of them now
I'm still trying together that answer my sell. So far the one I have are contained and would be easy to eradicated and I kind of like their look so they have not gone yet
Pseudocorynactis aka the ball anemone. Couple people have problems with them others say they dont do anything.i have a couple and they seem to only peek out at night.