Plumbing – Hot water is potable

I suspect you have a set-up like this?

You are concerned that when the water gets pumped back into the "water heater" it could possibly pollute your main water line?

The only thing I could suggest is putting a check valve on the "cold" water supply so that it can only supply fresh water. That is helpful if the pumps output pressure is higher than your supply pressure (This could happen on someday with low supply pressure or if you do not have main water pressure regulator)

In this situation the return is a dedicated port on the "water heater/boiler" and there should already be a check valve/pressure regulator/non return valve just before the water heater. So the rest of the house will run just on the cold water supply and never get any recirculation contamination.

Might not be, but looks like, effects of sulfur out-gassing from drywall.

See https://en.wikipedia.org/wiki/Chinese_drywall or Google 'Knauf Plasterboard Tianjin Co.'. Drywall commonly emits sulfur gas, however some of the lots from China were apparently really bad. The bad drywall was widely distributed, including at Home Depot.

Note: A lot of Home Depot Quality 'stainless steel' is not very stainless. But your picture shows a situation well beyond that. Everything under that sink looks 50 years old: note the corrosion on the pipes and valves as well.