Spotted turtle shell issues

[SteveW]

Anybody ever seen this kind of marking? I'm inheriting from a friend and concerned about a shell rot situation. Water has been highly filtered and keep acidic.
View attachment ImageUploadedByTortoise Forum1439267241.801219.jpg

 

[Yvonne G]

Hard to tell by looking at a picture, but to me it just looks like he's rubbing the shell on something that is wearing the pigment off.