crabs!!

hello everyone! first time on here been reading your advice for a few months now ..im starting up a new marine tank i think its about 200l and i have just added 15kg of live rock and its been a about a week and my tank is still cycling i have brown algae all over my tank it was worse but its getting better anyway my question is i have a rather big crab in my tank guess he come with the live rock is he any harm ? i have a couple of big bits of pulsing xenia on the rock i cant upload a photo or a link to the photo but as i need a bigger post count