What is this thing? Should I remove it

[Myka]

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).