Some kind of wood fungus. In Asian markets you can find many kinds of them, in black, white and other colors.

Many Sea Anemones live this way, and hold water within their bodies, squirting it out when squished. They often live intertidally, and "flower" when the tide comes in. One of the best places to find food for organisms that are fixed to the bottom happens to be rocks and other things in the zone between high and low tides. I would doubt that it is a Tunicate , or Sea Squirt, as they have two siphons, and this thing has only one pore. I have seen many of these on the Georgia coast.

It's indeed a Harvestman, or as we call them here in the South, a "Daddy Long-Legs". They are arachnids, but are NOT spiders, and are completely harmless, even beneficial because many species eat mold and mildew.

I already checked into how to separate isopods from the non-worm compost for exactly that reason. It seems easy enough. In the worm bin, I've noticed they tend to loiter near the surface; they aren't really found in the casts. I'm wondering if simply crumbling the stuff while applying to spot and pick out the odd stray will be sufficient. Alternatively, I might try a quick dunk to send them running for fear of drowning. I could try the spread and sun dry approach that's supposed to work with regular compost, but isn't it bad to let your vermicompost dry out?