Can a bike wheel be damaged if it's hit by a mop?
I doubt the janitor could unintentionally damage the wheel with the mop by impact -- you don't use much force when mopping, and it would likely just turn the wheel or shift the bike a bit if it was hit. If someone drove their boots into the wheel or something, that could cause damage. 

One concern is the chemicals used in the mopping. Certain chemicals (e.g. ammonia) can attack rubber and cause it to degrade prematurely. Depending on the concentration and amount of use, this may be a non-issue.

However, if you are worried about your bike to this level, consider storing your bike elsewhere.