Ya you have too high of water. A snapping turtle should be able to sit at the bottom of the tank and reach the surface for air. If you find that the water is too low for the filter to work properly, raise the bottom higher with more stones or whatever you have for a substrate.
Also, you do not have to provide a land area for your snapper per say, but it is recommened that there is an area where he can climb out of the water and let its shell dry. Be careful! Snapping turtles are amazingly good climbers, make sure there is no way he can escape. Mine even find a way to climb the filter when I put a rock too close to it.
For my snapper's basking area, I used three rocks. Two that are relatively the same size are on the bottom of the tank and I put a long skinny rock resting on top of them. This created a bridge that that my snapper climbs onto. Also, the bridge serves as sort of a cave area that my snapper loves to sit under. I assume it makes him feel protected. I would definitely suggest having a similar area that your snapper can hide under.
In addition to a filter, I would suggest getting a UVB light, unless the tank is right next to a window that gets sun during the day. Snappers usually dont actively bask, but it is a good idea to keep your snapper healthy.
Lastly, I would not worry about the white stuff on its bottom. Snappers eventually start looking like they are outgrowing their own shell, almost appearing fat, and white skin starts to form around its legs and bottom of the shell. My snapper also goes through stages of not eating, it seems to me that he is rather picky at times. If supermeals arent appealing to him, I will try crickets, if that does not work then I use some Reptomin turtle food.
Have fun with your snapper, they are great pets.
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Mike
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2 Painted
1 Musk
1 Common Snapper
1 Cooter
1 Map
1 RES
1 Corn Snake
1.1 Water Dragons
2 African Frogs
1 Toad
1.0 Degu
1.2 Chihuahuas
0.1 Cat
