Urgent Care :: something up with my map turtles carapace

[Sawback90]

So I just recently reaquired my turtles from my ex. Originally when they were in my care I took very good care of them well for the last 2 years my exs mother had them. Upon their recent return to my care I noticed my 3 maps have some sort of build up on their shells. Its kind of a off white color and its a lil crusty looking. My eldest female was in the process of shedding her scoots and I noticed where she had shed it was free of this build up. So im curious if its just like mineral build up or is it something I should be more worried about? Currently in the process of trying to get them and the tank healthy again so any advice is muchly appreciated.

[Sawback90]

I should also note my female RES is in the same tank and does not have this on her. But my other two maps who have not shed have their entire carapace covered in this, whatever it is.

[steve]

White coloring is usually fungus or mineral deposits. Can you post a picture of the area when it's dry?