On a sandbank in a harbor; some sort of worm?

was in a hole in a native tree that was being thinned out of a garden.

Boot for scale

Really strange : not segmented like a worm, not flat like Platyhelminthes, but moving (thus, not a clutch)

under rock in intertidal zone

Under a tiny seedling pot (about 1 cm). When pressed, brow substance came out

on a wet bank I'm guessing algal but also wondering about snail/slug eggs

In horse trough