new creature in my tank (uninvited)

[Ryan Joel Smith]

Update*

So i went home yesterday to see that my timer on my lights was not working so the lights were off for a 24 hour period which nothing in my tank was too pleased with.

BUT i noticed the top fin on my large female clown was bitten off. as well my much smaller male clown had one of his fins nipped as well.......Thinking this seems to be more like the two fish fighting as they are new to eachother then the crab doing the damage. Any thoughts?