What is this white stuff ?

[Koty]

 

[208reef]

Looks like your coral spit up its guts due to something that irritated it. Could be something as simple as a hermit crab walking across it or even a water change with minor difference is salinity or temperature could do it. Given the coral behind it does not seem distressed I suspect it’s just an acute temporary stress that should clear up overnight. Doesn’t hurt to check all parameters to be safe though and make sure it’s not a warning sign of something starting to get out of line parameter wise.