what kind of worm is this? north shore (above cape cod) MA

found this guy in the rocky intertidal, under larger rocks. north shore of MA, about 3/4 of the way to low tide so not all the way out but pretty decently low. i assume some type of flat worm, though worms are far from my focus (mostly algae) so i could be wrong. wanna know genus and species if at all possible. and i found 2 of these guys for the first time today, i spend a decent amount of time in rocky intertidal zones and this is a first for me.

won’t let me post a video, but he spat out some really long tube looking thing and sifted it through the sand.