What's wrong with this shell?

[KuroKyoto]

My mystery snail has a weird shell thing going on. This brown spot:

It is not as deep as it looks it's just really dark brown towards the middle area.
Is this a calcium deficiency? I have looked for cuttlebones and haven't been able to find them. I feed them algae wafers. They are not at my house so I do not have vegetables to feed them. I am moving them this week though to a house with a readily available source of kale. Would kale help any? Or would they even eat kale?
Also I just tested my water and it is hard apparently but I thought that was good for their shells.

[lohachata]

petco has cuttle bone...5 for $4.00...so does petsmart....pet supplies plus and most other pet shops...you can also order online.

[Obsidian]

Secure the cuttlebone to the tank, a suction cup will do, and the snails will eat on that and up their calcium intake