To me it kind of looks like majano anemones. Do they sometimes have a small bubble tip? They are mainly considered a pest as they can sting and grow pretty fast. I have them in my tank and every now and again I have to scrape them off. If you can keep them isolated you could actually keep them since they do look nice.
they move too, one is on the front of my tank. Thanks for the info, I was excited they might be something good...I also have green bubble looking things and I was told they are not good as well...
I just bought a creme brulee torch just to kill those bastards. Joe's juice also work well with them. However if you only have a few you should just use tweezers to get rid of them. Don't be like me and wait half a year and have 1000 mojanos covering your whole tank.