Metal ions in water can react with soap to form what?
"Scum" - precipitated insoluble salts.
It depends what metal ions you're considering, of course. But If there are calcium or magnesium ions dissolved in the water, and soap is dissolved in the water, you find that the sodium ion at the polar end of the soap molecule is replaced by a calcium or magnesium ion, and what results in a insoluble fatty acid salt. This can be observed as grey deposits on the sides of the container, which is colloquially known as "scum".
It is common in "hard water" areas.