Yeah, that looks like damage from low Ph and Kh. The crushed coral will help a lot as will adding calcium to his (looks like you have a boy there

) diet. I've been recommended the cuttle bone as well. I just got two tonight but I will warn you, its a pain to keep at the bottom of the tank! I'm actually going to post a thread to see if anyone has suggestions for that so keep an eye out.
Here is that list of foods that have calcium in them that your snail can eat:
The numbers beside each food is the amount of calcium per 100g serving:
Dill Weed (208mg)
Turnip Greens (190mg)
Collards (145mg)
Parsley (138mg)
Kale (135mg)
Watercress (120mg)
Beet Greens (119mg)
Chinese Cabbage (105 mg)
Mustard Greens (103 mg)
Chicory Greens (100mg)
Spinach (99mg)
Okra (81mg)
Leaf Lettuce (68mg)
Cilantro (67mg)
Purslane (65mg)
Endive (52mg)
Swiss Chard (51mg)
Broccoli (48mg)
Cabbage (47mg)
Rutabaga (47mg)
Brussels sprouts (42mg)
Celery (40 mg)
Sweet Potato Leaves (37mg)
Green Beans (37mg)
Romaine Lettuce (36mg)
Seedless Raisins (49mg)
Orange (40mg)
Lime (33mg)
Blackberries (32mg)
Kiwi (26mg)
Lemon no peel (26mg)
Papaya (24mg)
I didn't list everything I have on my list but kept to the higher calcium foods.
You can get a liquid calcium supplement here:
http://www.thesnailstore.com/index.php?act=viewCat&catId=9
As for your snails shell damage:
I am pretty sure you have a gold mystery snail there. The white area is damaged thin shell that should be gold. It won't ever be gold though. However, if you continue to supply you little guy with calcium and a good ph then all new shell will be healthy and thick
