I've pretty much given up explaining everything I see happen in pools. Sometimes, years later, I find out what was going on, but mostly not. Weird stuff happens from time to time in pools, and unless your goal in life is to be an amateur chemical detective, it's better to let it go UNLESS it's causing a real problem. Who knows, if you have a dog, maybe this:

is the explanation.

If you are regularly adding Jacks Magic, you may want to get and use some CuLator packs. They are not effective for quick clean-up, but they do continuously remove metals from the water, unlike products like Jacks Magic which work temporarily to keep it soluble. If you do get some, do NOT throw them away monthly, like the company encourages. They are not 'used up' till badly discolored OR fouled with goop. Keep one in your skimmer, but use a skimmer sock on top to protect the pack from 'goo'. You can also just chuck them in the pool. The more you have, the more effective they'll be. Just don't put them where they will get ruined with greasy dirt. (So, don't put them in a chlorine floater, where lotion and suntan oil can foul them.)

1 ppm Culator
1.5 ppm Culator, sold by Amazon
4 ppm Culator, sold by Amazon
Poolmaster 16242 Poolmaster Skimmer Basket Liner