What is this rustic water come from--New tankless water heater

Hi, Thanks for everyone ! The problem was gone After replacing the Union with tankless valve kit ( all are mady by brass) The problem was generated by the Union even it does not directly touch the copper pipe, but the water conducted the elelctrolyte-ion migration and the electricity (tankless water has electricity connected)accelerated the electode action.

I'm glad you figured out the problem.
I quit using dielectric unions years ago. Some cities around here like to use a 6" brass nipple between galvanized pipe can copper for that reason. I've found that a galvanized dielectric union takes very little time to close up when attached to copper.