That looks like mold.

You do have some kind of moisture problem. Don't just paint over that mold. I have used a product called Concrobium to kill mold, then when dry, painted with Kilz. I have read that bleach does NOT kill black mold tho this article does suggest it.

Cleaning black mold

Time for some selective demo. You absolutely need to find the source of the moisture and how bad the mold is behind that wall substrate,

There is no point in painting over that corner until you solve the reason for the mold. Don’t hide it. Grandma was right- a stitch in time saves nine. Fix the big problem first. Find the source of the water that is inside your wall, fix that, then repair the drywall and finally, paint. If the leak isn’t fixed, the wood studs will rot and cause a bigger, more expensive, problem.

" If the leak isn’t fixed, the wood studs will rot and cause a bigger, more expensive, problem "

Or the black mold will just continue to spread with the moisture until the mold becomes a health issue.

Notice that the mold (likely mold) is not just on your wall but also on the wood baseboard. When you are done remediating the issue causing the water damage, be sure to kill the mold on the baseboard or cut a section out and replace.

That certainly looks like mold to me. I’d stop the painting and have this examined and remediated. Mold can cause a host of serious health problems. Depending on where you live and your home insurance, mold can create a problem insuring and/or fixing your home with insurance funds.

Definitely mold. Paint wont cover it anyway, it will bleed right through. Cancel your painter, until you have found where the moisture is coming from. Thats a good sized area too. I also noticed it on your baseboard. The fact, that it wont wash off, means, it is all the way through that sheetrock, or plaster, whichever you have. Its also on two walls, so whatever the room is next to it, I would be checking that corner in there too.

Show a pic of the exterior. From a big enough distance that the roof can be seen. And from a close enough distance that we can see any changes to that corner.

Thanks everyone. I may have the painter paint everything else but leave that corner.

The thing is....it just wipes off the baseboard...no problem....baseboard then looks fine.

And the back of the paint looks fine.

You would still have to clean it with a mold disinfectent. Paint will not cover mold. It will bleed through. Even after cleaning, you need a sealing bonding type primer, or shellac over that area.

Thanks again all. Bleach does clean it. Painter will double clean and seal. I’ll get the mood checked too. Thank you!

You neded to find the source before doing anything further. You said this is an outside wall, perhaps there is a roof leak, window leak, etc.. You may have mold behind the drywall. Black mold is nothing to sneeze at. It will continue to spread with a moisture or rot source.

Don't bother painting it. You need to open up that whole wall until you find the source of the leak, probably the roof or a window (but who knows?). Water can travel sideways along joists before gravity brings it down.

You can go buy a moisture meter at a big box store and see if you read moisture around the area and on the ceiling.

You need to know what you're dealing with. It could be straightforward, but you may need a roof repair and expert opinions.