Yellow powder in fresh water tank vent tube

All I could afford wrote:
I’ve owned my 1999 trailer for 8 years now. I start every spring with a flush out of the pink antifreeze in the plumbing, followed by a sanitation of the fresh tank and pipes with a quart of bleach mixed into the full 30 gallon tank.

Last week when I went to service the system, I noticed my half inch diameter clear vent tube was clogged with dry yellow powder for the first 2 inches coming from the tank. I removed the hose and tapped the powder into a white coffee mug before washing out the tube and reinstalling.

Could this be from filling up the tank in mountain areas with high mineral well water? Should I be concerned? I’m having trouble trying to attach a photo today

I'm interested in the residue too. I find it in my fresh water tank drain, and also the aerators of the faucets. It almost looks like the spray foam that is degrading. But I don't know how any spray foam would have made it into the tank.