Could it have been picked up by the previous guests crabbing? I've no idea how crabbing works.
And I hate to say it, but the same thing happened to us. Except we were on the 17th floor!
We came in from the beach and suddenly noticed a crab halfway up the wall, holding on to a picture frame. Grossed me out like a giant spider would and we flapped for a bit, terrified it was going to scuttle off somewhere.
We went to reception and tried to explain to reception but with the language barrier they just looked confused (obviously). Eventually they sent an absolute tank of a man upstairs who came up the lift with us in total silence, but with an air of 'I'm here to rescue the poor stupid tourists' 
He was absolutely baffled when he saw it. Spoke no English and started mumbling and shrugging and looking bewildered. Eventually he just picked up a flip flop and just bashed it while we squealed.
At the time we had no idea how it got there, but now I assume a previous guest had put a dead crab there (judging by the way it pretty much disintegrated with one smack) and we just hadn't noticed it.
The next day I told the story to a hotel rep I was friendly with. Again language barrier. I said a crab was in our room, and he kept saying, 'a crap? a crap? how did it get up there?!' I shrugged and said, 'I dunno, it probably rode in the lift', which just made the whole thing funnier.
Sorry, this was just too weird a coincidence not to tell the story! I reckon a live one escaped from the previous guests 