Sealed lead acid batteries, especially those with gel based batteries, have the possibility of acid seeping out and causing corrosion to the materials in the surrounding areas, including the case. As such, batteries with cracked cases should always be replaced immediately.
Although an SLA (Sealed Lead Acid) Battery does not leak acid directly, there is a risk that its life-cycle and capabilities will be reduced if the battery ages. Acid may eventually start seeping out and cause corrosion to the surrounding materials, especially with gel based batteries.
There is also a chemical hazard because the elecrolyte is corrosive and contains lead. You should try to keep that crust off of anything you care about because it may absorb a little moisture from the air and dissolve or damage whatever it comes into contact with. Lead acid batteries use an acidic electrolyte (sulfuric acid).
Try to avoid running the battery down to zero. Sometimes, lead acid batteries can suffer from irreparable damage that cannot be fixed through reconditioning. One common cause of irreparable damage is sulfation, which occurs when lead sulfate crystals build up on the battery plates over time.
Nowadays modern plastics are impervious to acid so there is no risk of this happening. Myth: It is okay to store lead acid batteries anywhere inside or outside. Fact: It is good to store lead acid batteries in cool places because the self-discharge is lower but be careful not to freeze the battery.
They will make sure it gets recycled. Lead acid batteries are very recyclable (people will even pay you for old non-functional lead acid batteries). If you just want to get rid of it, you can probably drop it off anywhere that sells lead acid batteries (call first to make sure).