Scum is basically a sticky white substance which is insoluble in water. It mainly formed due to the reaction of magnesium and calcium ions with the sodium and potassium salts of carboxylic acids. when this reaction occurs,an insolube white precipitate is formed which is commonly called as scum.

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The scum is a Curdy precipitate formed when dissolved in hard water

scum is the product formed when soap is added to hard water due the reaction of soap with calcium and magnesium ions.
Scum is the product formed when soap is dissolved in water

Soap does not give foam with hard water due to the formation of scum. The scum is formed by the reaction of soap with calcium and magnesium salts present in hard water. Scum is an insoluble precipitate of calcium and magnesium salts of fatty acids formed by the reaction of soap with hard water. Soap + Hard water → Calcium salt + Magnesium salt

Hard water contain ions of magnesium and calcium reacts with soap to form insoluble precipitates called as scum…..?
Soap scum or lime soap is the white solid composed of calcium stearate, magnesium stearate, and similar insoluble compounds that results from the addition of soap and other anionic surfactants to hard water. Hard water contains calcium and magnesium ions, which react with the surfactant anion to give metallic or lime soaps:2 C17H35COO−Na+ + Ca2+ → (C17H35COO)2Ca + 2 Na+In this reaction, the sodium cation in soap is replaced by calcium to form calcium stearate.Lime soaps build deposits on fibres, washing machines, and sinks. Synthetic surfactants are less susceptible to the effects of hard water. Most detergents contain builders that prevent the formation of lime soaps.