poluted water cleanup

This is normal behavior.

Yes, it is odd, but any liquid that is less than about 20kg per tile that is adjacent to a solid tile can be assigned to be mopped up, even if it is at the bottom of a pool of a different liquid. It's actually quite nice sometimes, for the exact reason you encountered, cleaning vomit water from your clean reservoir.

Liquids (and gasses) don't mix in a single tile, so the heavier liquids will always sink to the bottom.