Red residue in saltwater storage tank

I got a food grade storage barrell of 50 gal sw, well I opened it today to find red residue all over the walls not heavy but enough to concern me. Any ideas what this might be and should I use the water? I cleaned the barrell with vinegar and ro I don't understand why its done this, could this be silicates coming off of it? It's a dark colored barrell it shouldn't be getting any light, your ideas would be much appreciated.

 

any help?

bump

 

There are clays that will build up from the salt mixes. As long as it doesn't smell you should be fine. I clean mine out every couple of months just to be on the safe side. (w/ vinegar)