any ideas what this is?

[IrishReefer]

Its what you think it is.... Aptasia or glass anemone. How long has the tank been up and running? They are real buggers to get rid of - that one looks like its base is beneath and in between the two rocks - Is it accessible for a syringe in the base? If not there are other ways... One way is to use the underwater putty stuff and seal it in. Or a Cobberband butterfly will sort it or peppermint shrimps though in the past this hasn't been too sucessful for me.

Only one thing is for sure.... it will spread if you don't take care of it.

Best of luck,

Brian