I had Slime back up bad in the valve stem and then harden. The only way I was able to deflate the tube was to actually destroy it. I've switched most of my bikes over to Air-Loc now, it doesn't back up into the valves and it doesn't harden. It's the "next generation" Slime. Also, I was told by my LBS to make sure when I parked the bikes to have the valve stem up so the Slime couldn't collect in that area. Unfortunately, I don't think you can mix Slime and Air-Loc, but it might be something worth looking into next time you switch out tubes.
OH: on the road bike I run Air-Loc tubes And a teflon strip just to be safe. (kinda like a belt And suspenders!)
I forgot: my LBS suggested putting the tube in hot water to help loosen the Slime if it happened again.
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"