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The picture isn't great but it looks like a messy spackle/mud job to me

Thanks. Yeah, it doesn’t look “alive” but with the roof link I was a little worried

Spray it with a bit of bleach and see what happens.

No, this is Patrick

Patrick? Do I know you

You ever been to Bikini Bottom?

it is hard to tell, but it doesn't look like it

No that's not mold, mold is more of a gradient texture or even blotchy, not just a pure black like that.

Thanks!

Are you talking about the black stuff? I’ve never seen mold grow like that.

Yes, thanks!

That's not mold, it's duct sealant.

Thanks!

Sure looks like duct sealant on a return to me too.

Vinegar to kill mold. Bleach won’t kill it.

No mold—dust particles pile up like this due to airflow.

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Thanks!