What's the difference between Crucial's Mac vs. non-Mac DDR3 SO-DIMM?

Good question. The short answer is that there is no difference in performance, since both will work. I know because I bought the same exact "non-Mac" kit, and I've had no problems with it. From a technical perspective, there isn't a difference between the kits. As to whether the "non-Mac" kits contain halogen and lead, I can't verify, but it shouldn't affect performance.

I don't have both modules, so I can't check what the differences are (if any). Most likely, it's just marketing from Crucial. The "Mac" version retails for an additional $10, so those who aren't tech savvy might be inclined to go for them, but otherwise they're the same thing. You can find more info on the types of supported memory for each model on Apple's support pages - this is the one for the iMacs: https://support.apple.com/en-us/HT205040. As long as the memory you're getting matches the specifications defined by Apple for your model, you should be all set.