It's expelling zoo, like the answers above.
LPS do this to control it's zooanthellea (sp?) algae within tissue. Have you recently acquired this guy?
Most do this if there is a shift in light intensity IME. Have you run or exchanged carbon? This could clear the water so the coral is recieving a higher amount of light.
Have you moved the height of your lighting, changed bulbs, added bulbs, moved the coral to a new location, changed LR locations above so it's not as shaded as before? All of these can affect the amount of light the coral is recieving. Then the coral doesn't need as much zoo to produce the amount of food it needs. So, the coral expels this.
They can also just do this time to time. But, if your other corals are looking puny then definately check your parms and report back. If you still aren't comfortable then do a water change and see what happens. :wink: