Yes, magnesium makes soap scum! From Wikipedia's soap scum page:
"Hard water contains calcium or magnesium ions, which react with the fatty acid component of soap to give what are technically called lime soaps"
If you like the idea of putting some kind of salt in your soap, I highly recommend salt soap. It's the only soap I make now, the lather is super thick and dense and it just feels extra luxurious somehow. My recipe is just 100% coconut oil at 20% superfat with salt and oil at a 1:2 ratio, i.e. one pound of salt for 2 pounds of oil. You can add extra fats like shea butter if you want, too, though.