If acclimated properly, mollies can really be in FW
, and brackish. How old is this tank? Were any fish recently added? You could be going through a mini-cycle if the tank is fairly new and new fish were recently added. Another reason could be that your substrate was not thoroughly rinsed and you may have kicked up some dust. I do want to warn you that damsels don't do well with other species of damsels, so don't go mixing them or you will have a nasty outcome, this includes clowns.