Can anyone tell me what this is?

It is a type of green bubble algae. I had it around some polyps and it almost choked them out. It is pretty invasive and most people recommend removal.

Careful with removal though because you don't want to pop the stalks or else you'll release spores that will spread the bubble algae to other locations in the tank.

If you can, just take the rock that it is on out of your tank and scrub it off in a seperate bucket full of saltwater, then put the rock back in. That should help prevent it from spreading elsewhere in the tank.

Okey, Thank you all =)