Salt in water softener has grey/black "dirt" on it. WTF?

I have a Kenmore water softener. I lifted the lid today to look at the level and was surprised to see this grey stuff sprinkled over the top of the salt. I pulled some out to look closer but still can tell. The only thing I can think of is I pulled (ok, busted) some drywall off the wall next to it in the garage. Is it likely this is drywall dust that drifted in, settled on the salt and got damp? Not sure if that would turn it grey. The lid on this thing is definitely not close fitting or air tight. I'm wondering if I should try to remove the top layer, or will it matter?

 

Yup, looks like drywall dust. Probably won't really hurt anything, but may clog up something. You don't have a cover on the bin?

I'd dump it out and hose it off. The salt you'll waste is worth the peace of mind.

 

There is a lid, but it's not tight fitting. Think of a big garbage bin- the lid overlaps the container but doesn't seal it. I can see fine dust in the air making it's way in. I removed the top layer of salt and that got most of it.

Thanks!