I doubt it's 'active', but it's probably a solution of either calcium chloride or calcium carbonate - mainly water!!
Have you actually tried seeing how the snails do without it? With plenty of greens and a good diet I never found any of mine to need anything extra, even in soft acidic water where they were growing their shells.
They're suffering quite a lot of pitting since I've moved house, inverts in my tank are generally not doing well (see this thread for details) so while I never needed it before, it appears to help a little now.
They don't get many greens but they do have a varied diet along with my fish so I don't think that's the issue, it was the same as previous... though now we're into spinach season I might put a little of that in for them
I guessed it was mainly water, I was just curious as to what was the non-water component.
In a previous thread on a different subject James said that calcium carbonate was 'virtually insoluble' so maybe it's calcium sulphate or as you suggest calcium chloride.
Dennerle do a similar kind of calcium product for shrimp, although I'm not sure how economical it works out. Off the top of my head, it's 5 drops per 20liters - I've been using 5 drops in my 20 liter ever week, and I would say I've used half a bottle in about 6 months. I use RO water and I've never noticed any pitting in the shells of my shrimp