Help! Mystery creature. Puerto Rico

Submerged live rock from Puerto Rico. Was out of water for month, exposed to freezing temps. After placing in otherwise sterile Saltwater aquarium, all was normal, now one more month has passed, and there is some creature alive inside the liverock with what looks like a proboscosis similar to that of a conch snail, sticking out of the rock moving.

When I poked it, it sucked back in as much as it could and now looks like a closed zoanthid. However if I shine the light inside the rock I can see the flesh tube goes almost an inch into the rock also looks like there is a small white hole or siphon in the tube or body it connects to but can only see it through a pinhole in the rock. I don’t see any shell, or how a shelled creature could fit inside the rock.

What is this thing? Some type of clam, ribbon worm? Snail? Help!