Gray stuff in tank I use to make saltwater

I use a 29 gallon to make batches of saltwater. In there is a maxijet, heater and another pump\powerhead that has an air intake and provides air bubbles to the saltwater.
Everything gets caked with a hard gray substance. It looks filthy. Does anyone else have this, and is it good or bad? I scrape it out once in a while.

 

I had a friend who had a 50 gal trash can do the same thing. He ended up replacing it.

 

I thought I had read awhile ago that it is clay. It is a residue that is leftover form the salt mix. I don't remember where I read it, sorry.

 

I've always heard it is mineral deposits.Don't know if that is true or not.When my make up can get nasty I fill it with water and about a half gallon of vinegar.I let that run for 2-3 days wash it out with fresh ro water and let it dry for 2-3 more days.Always works great for me with no ill effects.