About the size of a dime...

[Richardsonic]

I have three or four of these that came in on a couple of rocks. They haven't seemed to spread at all in the two or three months since they've been in my tank. Just curious about their identity and if they're a cause for concern. Mojano or something else?

 

[littlebeard]

Looks like Aiptasia or similar pest anemone.

Check out the hitchhiker guide

 

[ChiCity]

majano
(pest)

 

[Gtinnel]

I agree that it looks like a mojano

 

[Richardsonic]

Thanks folks.