I, via this medium, cannot tell what you actually have, some one needs to come out to see it.
But, before I do that, I would stop the leak from the water heater and then ventilate that cupboard somehow.
Maybe even to install a vent system in the door about three inches from the floor. You can get a soffet vent or some decorative vent to use on the door to make it look nice and to be effective.
Clean the mold with bleach and water and stop the leak and vent the area and see what happens.
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