can black mold grow in humidfiers?

i worked at a place as caregiver. the room humidifier in one of the clients room was nasty. didn't want to do it. took it out and thought i would clean it and add the bacteriostat. lifted up the main component and was shocked. it had a black slick on it. against my better judgment tried to clean it. difficult to remove. noted the inner fan parts had the same residue. i gave up. did not set this up for the client. i did report this to the supervisor. checked it few day later and clean in the reservoir. still noted black residue in fan component.in use now. could this have been black mold? over reacting. since resigned from position as too many "iffy" situations. thanks to all sincere answers.