Catch the tiny one and look at his claws. If the claws have a space in the middle even when closed and are rather flat, it's probably a porcelain crab. Watch it and see if it ever starts to fan little things out from its mouth, that means it's a porcelain and is totally harmless.
In fact... It seems to have its fans out. I'm pretty sure that little one is a porcelain crab, now that I've looked.