I have some cellulose deposits in a line between the toilet and the vanity in a bathroom. I remove them and they come back sometimes days sometimes hours. It is non metallic although it has the appearance of rust. It is always in a straight line.
I have pictures but can not post on this site.
Not sure how to look at this, exterminator, plumber, carpenter, etc.
Any information would be appreciated.

What is a cellulose deposit? Never heard of such.

Efflorescence I bet.

Limescale, calcification, mineral deposits, etc.

Use lemon juice or vinegar or products made for mineral removal. Is it in the edge of vinyl tiles?

You can post pictures, click on the ,"Go Advanced" button.