Sounds like iron in the water....

Have it tested, there are number of iron form: bacteria, compounds,
etc.

But first step is to take it to the lab for testing.

number of solutions available, such as chemical treatments that injects
chlorine or ones that use some catalitic reaction on filter, and then
wash away iron with system backwash cycle.

see:
http://www.reopure.com/terminator.html

Anyway do your own research.