Looks to be a ball anemone to me. I don't have much experience excepts for kalk'ing them. MIxed info on the net as some say they are fine/non-invasive others say they are invasive. So....ya thats my $.015
thanks I think that's it. From some quick google searching it looks like they are not invasive enough to cause problems, slower growing, small and tend to stay in the dark areas. So I think I will leave them be and see how it goes for now.
Can you take another pic with your blues off and whites on if it doesnt look like the one below I edited for you for easier detection. It looks like either a ball anemone or a ricordia. Either way, you should probably kil it with kalk paste before it spreads.
I have a few balls in my tank and never had an issue with them. they stay small havent spread much and mostly stick to the underside of rocks, so not interfering with any corals or anything