What is this thing? Should I remove it
 

Blurry photo but I think its good enough to see.

This has been in my tank for awhile but seems to be multiplying now?

So any ideas what it is?

Should I leave it or try and remove it?
Thanks

they look like zoanthids

possibly mojanos

Those are Common Green Button Polyps. "Reef-safe", but can multiply uncontrollably and sting corals it touches. Do a Google image search for "common green button polyp" you will see lots of pics of em.

Paly's are my vote

Thanks,

After some googling I agree with Button Polyps, it started as one and now there are four the biggest is the sizes of a quarter.

They will definitely multiply. If you can confine them to one rock on the sand that is the best approach. They can still "jump" off the rock, and attach themselves elsewhere, but a keen eye can prevent that. I have some on a rock in my tank, and every once in awhile I will find a polyp that has popped off the rock, and on occasion one that has made it to attach to the rockwork. I give those ones the ole Kalk paste solution (or Joe's Juice).