Any ideas? Mildew? Can’t be dust. I usually have high humidity 75-80% as the grow was in seedling stage, now I’m in veg stage so I dropped humidity to 60% , photo taken through screen. I rubbed the center, seemed to come off
No white on leaves.
Its dust my friend. Mine does the same. If the cloth would have been white then the dust would look more grey/dark. You can disconnect the filter during veg to be honest unless your girl is giving off dank/skunk smells already.
Do you use a misting humidifier? If so that’s where it’s coming.
I suppose that is possible too. My fogger never reaches that high though. Id imagine it would feel wet. @legalgrow can you take a dry napkin or something and rub it around and see if moisture gets on it?
I tried a couple of misting ones, ended up buying evaporative, our water is hard and was not buying distiled water to run in the misters. Was white dust everywhere but more on filter of exhaust fan.
So if you’re using a fogger or a humidifier and you’re not putting distilled water in it that is calcium from the tapwater I stopped using tapwater. I never had that white stuff again.
sorry, didn’t see the post above me but yeah, what they said
Save your carbon for when in flower and humidity is low and Smell is high 
Looks like mine, dusty.
Can’t you use the fan without the filter during veg?
You’ll get to flower and see the filter isn’t trapping smell because you used up all your active carbon with high veg RH
I rubbed it with my finger, it was dry.
I’ll say this but don’t tell anybody else…
I use tap water and dgaf about calcium build up.
If my Huey stops, I clean it with brake cleaner 
After a few years I replace it.
But I’m am a lot picker with my carbon filters 
Treat them with love and care, bag them when not in use 
If calcium you’d see the powder more places unless it’s not a lot. Dust is everywhere. But I can’t say with certainty what it is, but looks just like my filter sleeve which is dusty.
My girl stunk in veg so I did use my filter early on.
Maybe an air scrubber in your lung room would do ya better 
My temperature is rising without using the carbon filter. I need more fans. Just rose to 83F in few mins
So if you leave the filter on the exhaust temps go down, but without it and just the exhaust it goes up?