What is this yellow stuff?

[Flapper]

When replacing the leaking fitting connecting the HDPE pipe to the galvanized pipe, I took a picture of the inside and I found rust which is typical with galvanized pipe, but I also found this weird yellow stuff... what is it?

 

[Smooky]

It might be iron bacteria

http://www.health.state.mn.us/divs/eh/wells/waterquality/ironbacteria.html

 

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[Plumber69]

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