Mold behind vanity in bathroom

[Bienes Raices]

In one of my properties there is about 2 to 3 square feet of of mold on the base boards and the drywall just above the baseboards behind the vanity, under the sink's plumbing.

The visible parts of the plumbing are not leaking. I haven't seen evidence of mold anywhere else in the property.

I would like to fix this myself and avoid using remediation people if at all possible.

1) Is the best course of action just to cut out those parts of the drywall/board that have it, so I don't risk contaminating other areas, rather than trying to scrub it off? (and then replace that drywall)

2) Since there is no visible leak, does that mean that there must be a leak somwhere behind the wall? I don't see any holes it the drywall where mold could have spread from the inside of the wall.

Thanks.