Black dust formed on a heater, toxic?

[Fishingandreefing]

I can actually see the brown crud floating on my water. But as I mentioned earlier, it dissipates after 48 hours of mixing, but does stay on the sides of the barrel, and PH's.

Not really an issue with IO, but it is significantly more in reef crystals. I have also read it was an anti caking agent. Been using IO for years with 0 issue and the brown crud.

Oh I had those too with IO and that was one of the reasons I switched.