It's book lice - I think. I've found them running around the shells of my crabs years ago and they've never taken over or caused any damage.
Not in the numbers you are finding though. There's a good chance they're in the crab's shells for moisture rather than doing harm to the crabs. Still, it's a good idea to do the bathing for now. With the pointy abdomen and the legs I really think you're looking at a 6 legged insect and not an 8 legged mite.
Definitely get the crabs some turbos and they should change. Neither is a good shell choice for PPs. Kip is right that one is in a sinistral shell! Either you have a seriously unique crab still wearing it's original shell, or the poor thing is going to be frantic to change into a shell that goes the right direction. Once they change shells you should have a better idea how the infestation is.
Are they in a fresh tank? Offer fresh fruits and something starch based (cereal) as well which should lure the booklice out. They have pretty particular feeding habits.
Do you know how long they were in the dumpster? The only worry about the crabs I would have is that colder weather. Effects of being too cold take a long time to rear up.
