White mark under shell

Question
I have a baby red eared slider that I inhereted a couple weeks ago. Recently I noticed a white marking starting on his belly, in the middle. Top of the shell looks normal still.
He's still very active and eats fine--feeding him turtle pellets and dried shrimp mostly, doesnt seem to like frest veggies when I try them. The tank I'm keeping him in is smaller than I'd like, will be upgraded in a week though. He has a water filter, lights for heat and uv, and a basking area.

I'd like to know if this is an an infection I need to worry about, normal since he's growing, or what? Link below goes to a picture of the turtle top/bottom

http://tinypic.com/r/2f0drhx/5

Answer
Hello after looking at your pictures, al i see is where the cord was attached and just has not gained color at this time. It is a young hatchling, just lost its sack and will be ok. You can see that the area where the white dot is would be where the yolk would have been right after leaving the shell.
best to you
let me know if you need any more info