Cucumber,
In smaller setups with poor mechanical filteration, it can nuke a tank upon death. Atempting to remove it only stresses it out nuking in defense. If it goes on the glass or somewhere it'd be a good idea to snag him slowly.
Aside from that if you wish to keep him, they're benefical (Scavangers?)
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Off topic, TBS, that little white crab listed kinda looks like something of the calappidae family. Can't say which one exactly or even be 100% positive that it even is a calappidae sp. But for some leads thats my thoughts on the little guy.