What is the brown residue that builds up on my saltwater mixing equipment/buckets?

[AydenLincoln]

Whenever I make saltwater I always end up with this disgusting brown scum all over the bucket. I don’t store stagnant saltwater in it and whenever there is saltwater in it…it’s constantly being mixed and heated and used within a few days at the very most. The equipment I use is very old and dirty after it too. But I always make sure to rinse and scrub it with a sponge between uses. But does this happen to anyone else? What causes it? I use Fritz Reef Pro but I have used Instant Ocean and Red Sea and even different equipment and it happens every time. Is it bacterial? Salt residue? It’s super clean water before I add the salt and has 0 TDS so I believe it’s from the salt. Does anyone else have this happen?

 

[EeyoreIsMySpiritAnimal]

It's from harmless binders in the salt mix. Won't hurt anything.

 

[Idech]

The only time I had brown residue is when I mixed Red Sea blue bucket for two long (you’re not supposed to do it for more than 2 hours I think).

 

[AydenLincoln]

It's from harmless binders in the salt mix. Won't hurt anything.

I figured it was probably normal but honestly always wondered what it was.