Hi Jim. I am no mold specialist, nor do I play one on TV. So with that being said, unless my Air Quality Specialist, (Industrial Hygienist) identifies it as such, I would refer to this as a "fungus or "mold-like" substance.

I have seen a lot of "possibly mold" covered surfaces in my day, this is a different surface effect than normal, no visible spore-like formations, fur, growths, etc. It seems to be worse at the butt end joints? Where water may have seeped in and dried? It has that efflourescence look to it. May be time to call in a specialist to determine if this is indeed a harmful mold. Unless you are a qualified, certified mold inspector that is. If you are, I defer.

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