Looks like possibly aptasia anemone. You probably want to remove it. I've used the lemon juice method and joe's juice to kill them. Both have worked. Since then...my peppermint shrimp keeps things under control.
There's so many options and google will come up with a ton. Here's a couple
1) Lemon juice. Put a tiny bit of lemon juice into a syringe & inject it into the base of the anemone. It'll shrivel up right before your eyes. Be careful on how many you treat as it could effect your ph. Do some, then wait a couple of days. I tried this....worked fine. Contrary to what I've read, it's been a month and a half and I haven't seen new ones everywhere.
2) Joe's juice.. I ordered online. Only used it once for one anemone. It was easier to use as I didn't have to "inject" it...you only "feed" it to the anemone. But it came out in a big glob. However...it did kill the anemone & didn't effect anything else in the tank. Same concept, but more expensive than lemon juice
3) Boiling water. Same concept as lemon juice. Inject directly into the base of the anemone... but I haven't tried it myself
4) for maintenance. I would suggest a peppermint shrimp. I've heard they're hit or miss...but mine does a great job. Haven't seen any aptasia for more than a day or 2 and he gets them.
i've heard there is a specific type of peppermint shrimp that will eat them.....i just cant remember the exact name of it at the moment....would one of these cure my problem? or would it at least make them steadily disappear?
i found this:
Peppermint shrimp (Lysmata wurdemanni) are used by many hobbyists to eat this anemone. It can be hit or miss though. Especially if you get the wrong version of the "peppermint shrimp". There is another peppermint shrimp, Lysmata californica, that looks very similar to the wurdemanni but doesn't do the job on aiptasia that wurdemanni does.
is this a pretty easy thing to get locally and cheap as well? any thoughts on it