The second one looks like the ones I have and I have had no trouble at all with them. Mine eat algae and don't travel far from their hidey hole: they come out at night and scrape the algae away.
Did you notice any damages in your tank that might be contributed to them in the past?
Did you notice any rock work in your tank that had scraping marks on it? If so, I'd contribute that to the second crab.
I don't know about the first crab, but must say it has a lot of spines on it. This might mean that it is a more docile crab? Actually, it looks like a little mud crab, but for two years old it's probably too small to be one.
