The window frames are aluminum, not galvanized steel. What happens is that there is some water which gets under the coating (paint) and then corrosion. You are seeing Al oxides.
What to do: sand well to bare metal, be sure it is clean, then clean with acetone, and apply a strontium chromate (two part epoxy is better of you can get some. Then a coat of primer, and finally "Hatteras White" is a very close match for the original paint.
Yes, most of the boats have this in one or more places.
You can take the frames out, and send them to be powder coated if you want to spend some real coin. But this means removing the glass etc--and you are probably better off buying new windows..more coin.