What was being fed?
Kinda looks like it captured some food and is closing around it directing the food into it's mouth.
The mushrooms(?) on the rock in the background, also look like they are feeding.
Otherwise, what are the Alk and Ca levels? Between 8-14 dKH and 350-500 ppm?
If this isn't it, a full tank pic would be helpful.
Aloha