Help identifying something based on description

[kusanagiz]

I've noticed this very very thin white tentacle that would come out of a hole from a rock and like a tongue would lick the sides (or in this whip the sides) of the rock in an attempt to i guess collect food. Any ideas? it moves really quickly...definitely not a bristle worm.

 

[Len]

These are probably the tentacles of Spaghetti Worms. They are harmless detrivores/scavengers that are doing exactly what you're thinking: reaching out to collect outlaying food.