For me it’s mold
I will doLet it sit for a while. If it grows and spreads, it's mould.
Please define “plume”?@jeremyreeves Thanks for the information.
I'm not an expert but I find it nearly impossible to diagnose mold vs. plume vs. other deposits from unmagnified cell phone photos. I use the rule of thumb that under magnification if it's fuzzy it's mold, if it's sparkly (like sugar) it's plume. I don't smoke anything that has active mold on it, and if I'm not convinced one way or the other I won't smoke it either.
Now THATS mould.