Greenish gray mold on wooden window frame
You are probably getting this because of condensation buildup on the windows plus the wood to feed on at the bottom.  Probably no way to totally stop the condensation but simply opening the windows once a week or wiping off the window area with a towel would probably help.  Also turn on exhaust fans when taking warm showers or cooking a lot.

To clean just use soap or laundry detergent.  Scrub it with a foam pad and a wire brush for wood parts.   You will probably need to go through a couple buckets of water.  Once everything it is off spray on bleach to the area and wipe off with a towel.  Most of the time bleach will have little to no affect on wood unless it pools there but test on a small portion first if you want.