Remove dirt of mold from wall

Hi, i'm a new user of photoshop and i'm trying to figure out how to remove dirt of mold from walls in one of my photo.

So I have this situation:

And as you can see, there is some black in this grey wall. I already did some test with rasterize level and brush tool (photoshop normal not beta) and now i'm here:

Way better! Now, i want to get rid of that dark grey area and make it as the light grey in the down part of the image.

If I use the brush tool, nothing changes, how can I do? Thank you so much for your support!