I believe they are some type of anemone. The only thing bad about them is they multiply fast and I have heard they would grow everywhe re in the tank. I have also heard that they are harmless. I founf 1 in my tank recently and removed it..good luck..
Ball Anemone, they technically aren't a Anemone and won't sting other things, its in the Mushroom Family and is Reef Safe. Melevsreef.com - Orange Ball Anemone
I had several. They seem to come an go with the level of nutrients in the tank. At one point I was feeding a lot to my Scooter and the nems really took off. The fish died and I stopped the heavier feeding and the nems all but disappeared.
I won't worry one bit about having them in your tank. I have never in my 14 years of reeding seen anyone have them get even more than a handful in a tank