For me crabs are the number one hitchhikers I get. Luckily recently I got a small emerald crab

. A few weeks ago I found a crab that must have been in the tank 2 years since I hadn't added anything big enough in that long for him to come in. I'm pretty sure he is a baddy but so far he hasn't caused me any problems that I know of.
Something I was told is that if the claws are very pointy it's predatory and bad news, if they are blunt they likely eat algae. Of course I believe there are always exceptions, as I've heard even emeralds are a pain once they get bigger.
You probably already know this but looking in the tank at night with a red light can be very helpful, you may get a better look at this crab as well as find out if there is something else that could have ripped apart your cucumber and/or is killing your snails. I once found 2 very large fireworms and promptly removed them. I figure they were responsible for killing $500 in shrimp, hermits and snails. One very large snail was found half alive in the morning, full of bristles. He did unfortunately die.