I had a little hitchhiker in my 33g for a long time & it didn't seem to be doing any harm as long as it stayed on the bottom. One day I see him climb to the top of my LR structure & snip, snip, over the next week I lost my pulsing xenia colony that I had been nurturing from about 6 different frags
Never did get the bugger but haven't seen him in about 8 months now.
Personally, I go along with the innocent until proven guilty, but if I suspect a crab is doing damage, he goes in to my 20g Mantis tank
for final judgement
Anthony