I have crank out windows in both bathrooms.

This is usually caused by moisture, and must be scrubbed down. This usually happens when water is laying on the lawn, or extreme rains. Be careful if you are in this case. Some of our neighbors are in this same case. Be alert, and wise. Seek advice from Health Dept., just to be safe, to know your next steps, for your safety, ok? Best wishes, J.

Sounds like a moisture/ventilation problem. You can try using a commercial mold removal product, followed by a mold preventative product, or paint it with a paint treated with a mold retardant (Bullseye makes a sturdy primer paint that I have used and seems to work.)

I agree with Susan! You need to ventilate the rooms. Do you have exhaust fans in the bathrooms? If not you can crack open the windows slightly to dissipate the moisture. Moisture causes the mold. Once you have it cleaned, seal the wood with either a few coats of polyurethane or varnish.

agree with Susan. If the wood is still good might want to put a polyurethane coats on the wood.

Make sure to run those exhaust fans. Turn it on before you shower and leave it on until 30 min after you shower. if you don't have exhaust fans that may be why you have crank windows there. Do you open them when it's warm outside and keep them closed in the winter?