Saltwater buckets - dirty at bottoy

[FOBbit]

I can't make my own water. I live in an apt. No space. So I buy salt and top off water from my local fish store.
When I'm done with my water changes I clean the buckets out, and notice a grey-ish film on the bottom. I just wipe it off with a paper towel. Buckets are either old salt, or ones I bought from the fish store.

Question is why? What is causing that?

 

[Doctorgori]

well it’s not reef crystals salt cause that would be reddish brown