Valonia macrophysa (Sea Pearl)

Source reeflex.net

QUICK STATS

• Care Level: satisfying
  
• Reef Compatible: Yes, as long as it doesn't grow over Corals
  
• Max. Size: Up to 3cm
  
• Diet: Zooxanthellae / Light
  
• Origin:

This algea can be quite a plague, because it can spread very quickly.
It is often eaten by fish, mostly all doctors, but only if the balls are quite small. Also the Fox Face often eats them, but he needs a certain size aquarium.
Unfortunately it often grows in branches Acroporen which they don't like. It is hard to (completely) remove them, since you should remove them intact. Bursting them will cause spores to distribute through the aquarium.
The statement that it only grows well with good water conditions is believed to be incorrect. Often it does well in worse conditions.

Other links:

The whole aquarium is filled with these, especially near the incoming waterflow.