Mold

Thanks for reaching out! If there is mold on the seal of the window, it is likely that there is a leak in the window or some way that moisture is getting into that area.

I would be happy to recommend replacement weather stripping for your window. I'll just need to see a photo of the strip from an end profile view, which means you'll need to remove a piece from the window. I need to see the shape of the piece and you'll also need to be able to confirm measurements. There is a chance that changing out the weather stripping in that section could fix the issue but there is also a chance that it could only provide you with a temporary solution and the mold could come back.

There are several suggestions/remedies for clearing up mold, online. It might be a good idea to search for one of those solutions to see if it may help you with the mold issue. However, I would also suggest contacting a professional if the issue is of great concern to you as that isn't generally our area of expertise.