I recently had a case where after T moved out LL discovered mold. LL claimed that T failed to notify LL about it thus LL was claiming T's entire security deposit + more. At trial the Judge ordered LL to return most of the security deposit to the T. Does lease say anything about mold? When did you discover? When did you notify LL?
Yes. Civil Code sections 1928 and 1929 require tenants to properly maintain the rental and to repair any problems they caused. Mold on the ceiling is generally caused by the bathroom door being left closed in a bathroom that has no windows. To avoid mold requires only that the bathroom door be left open and that a small $20 be placed to exhaust the humidity. As to cleaning the mold, that's your call. Personally (meaning this is not an opinion expressed as either an attorney or any other professional) I don't buy into the bad science (BS) about toxic mold and how it will take out your entire family including pets if you disturb it. The "certified mold remediation specialists" who advertise their services are people just like you who took a 40 hour course and passed an open book exam. I would rely more on tips from good housekeeping.
A proper response would require a thorough investigation into the history and background of this relationship. The information provided above is just that, information, to be used as you see fit.