I give my little fishes a slice of cucumber every now and then. First I slice, and then I boil them to soften them up. I also leave the skin on to hold the slice/s together, but this is really up to you.
The bigger the chunk of cucumber the more chance it won't get eaten, and it'll go bad quicker in the heat of your tropical tank. This will pose a dangerous threat to your fishes if they eat decomposing food.
jaime007, I'm not sure you have any fishes that'll eat cucumber, but there's nothing wrong in giving it a try. However, your crab should enjoy it.