Not sure if the attached picture is clear enough, but I can see a slight gap in the frame where it has probably been leaking. Not leaking from outside, just the inside condensation seeping through into the sill.
Thanks for all the advice. Yeah, the photo is upside-down, I figured my phone would flip it. Guess not.
The windows aren't old, neither is the house, and it sounds like killing the mold with bleach and then maybe covering with Kilz before installing a new sill is the way to go. We're planning...
Here is an outside shot of the trim (Hardi). If it's an exterior flange nailed to the frame, maybe it will be as simple as removing the trim and window, repairing the framing, then putting it all back together? Any worries about damaging the aluminum frame?
I have a house that was built around 2001, and still has all of the original windows. The inside window sill started to swell from the condensation running down from the window, so I pulled it off intending to simply replace it.
I noticed that the framing under the window appears to be moldy...