Corrosion on bottom of copper hot water pipe from rust particles. This is from a recent repair by Massey Plumbing Inc.
Inside of pipe above
For many years copper was the gold standard for water lines, and is still sometimes the preferred pipe material. It is much more expensive than the alternatives, and can be corroded due to improper installation, water chemistry, electrolysis and rust from a water heater.
There was a leak on the cold water side of this shower valve