That's not good at all! I'm not sure how long the spores last myself, sorry...
I would return the stuff to those bums! Gads, that's no good! You don't absolutely NEED mycos, so save your receipt and return it next time you go to the shop, and grab a new can when it's more convenient.
Remember not to over-think the grow process. It can be as simple or as complicated as you would like it to be. ProMix is among the simplest media if you just make sure your pot size and your plant size always match well, don't overwater or overfertilize, and start out by getting used to growing with the basics- soil, water, light, simple fertilizer- before getting too fancy with supplements and bloom boosters.