dust-looking residue in bathtub

[Thanos]

Hi all,

I was wondering if someone can help me with this:
after I take a shower there is brown dust-looking residue in the tub. Any thoughts what is it? (I am attaching a picture)

Thank you

 

[johnjh2o]

Looks like rust from galvanized water lines. Could be just the nipple used for the tub spout.

 

[SHEPLMBR]

Do you have a water softener or neutralizer system?