Valonia sp.
To remove it I'd have plucked the coral out of the tank, and carefully picked them off, trying not to burst them.
Since that ship sailed, it's probably not a huge deal - if he spores were there, they were there. Emeralds, Naso Tangs and Foxfaces burst them while they eat them and it doesn't necessarily mean that a million more will grow back. When you see them pop up, carefully pick them off and remove them.
Jenn