The presence of mould may not be due to a leak although being under the bath makes that a favourite. I would expect to see water marks as well as mould, you did not mention any being present. Mould could be present simply through lack of ventilation and you have a cold spot allowing vapour to condense.

If it is a leak from the bath then the the most common cause is the seal between the bath and walls followed by loose piping either waste or to the taps. I would check out all of these before more drastic work. The bottom line is if a water leak is the reason for the mould you need to find it and sort before doing anything else, covering it up will not make the problem go away.

Richard