noun
Refers to the hard, chalky deposit left by hard water in kettles, pots, bathtubs, etc.
やかんに湯垢がたまったので、クエン酸で掃除した。
Scale built up in the kettle, so I cleaned it with citric acid.
お風呂の湯垢を落とす専用の洗剤がある。
There is a special cleaner for removing bathtub scale.
水垢 is scale from cold water, while 湯垢 specifically refers to scale formed in hot water.
Compound of 湯 (hot water) and 垢 (dirt, grime).