Some will (in)correctly identify these as Blue Clove Polyps. IMHO they are more likely Sansibia sp. Although they, like clove polyps, can spread quickly, they have no defense at all against other corals and will lose any battles for real estate. Personally, I like them for that very reason, especially in newer tanks where there is a bunch of bare rocks begging for color.
When I started up my current DT, I put a few small clumps around the tank. They quickly covered a large part of the rocks, but as the stony corals have grwon, they have receded to now where they are only on a couple of outcroppings.