could possibly be the start of shell rot or a type of fungus i'm not sure. its kind of too blurry to tell. Shell rot is usually brown spots though, although white can be it too. keep an eye on it, if it seems to be spreading or the shell is getting softer or has soft spots, look up shell rot/ shell fungus to find the proper treatment. its curable so don't worry
Could it be shedding ? Thats what it looks like . Is it flaking ? Does it have a place to get out of the water and dry off? What is you set up like temps basking,water ? Looks like it could be some new growth but can't be sure .