John's suggestion about the sand blaster and glass beads sounds ideal, because it really would clean the action, and "get the corners". If you don't have a compressor etc, however, cleaning by hand using any of the branded anti-mold products that you can get in hardware stores should be ok. I'd be inclined to wear a face mask if the growth is heavy. Although, if you are cleaning with a solution on a damp cloth, hopefully not much will get liberated into the air, and most will be taken up on the cloth. It's important to consider future prevention. For this mold to have grown, conditions must have favoured it. The piano must have been in a damp room, with moisture condensing inside it. Occasional use of the piano in a room that is itself only occasionally used, (and only occasionally heated/ventilated) are likely factors. It may be a good idea to suggest a Dampp Chaser. This will keep the piano warmer than the surrounding air in the room, so that moisture will not condense into the piano in future. Best regards, David Boyce
This PTG archive page provided courtesy of Moy Piano Service, LLC