It might also be a defensive response. How close is it to other corals? If you notice in the picture it is only extending the sweeping tentacles were it is next to another coral. I would watch out for some stinging IMO.
yeah i thought of that but if you look really close ... you can see the tentacles on all sides, just not as many on the left side. they were all over the back side of the coral and some on the left side, but kinda hard to see in the pic.
Repto called it out,they're called messenterial filaments.These are expelled by a coral for many reasons.It can be a response to food,aggresion/defense,or stress.Since Pachyseris spp. are nearly autotrophic relying on photosynthesis for most of its nutrition.My guess is that it was an aggresive/defensive response or stress induced.