White patches on laundry - could it be my Magnetic Descaler?

I live in a hard water area and I have been in the habit of adding water softener products (Calgon and the like) to my wash.

Last year I moved to a new build block of flats and I have discovered that everyone's water supply has a Liff Softest Magnetic Descaler fitted.

After I moved to this address, I kept getting white patches on my washing that would not wash out if I washed or rinsed again, so I figured that it was not the usual laundry product deposits.The stains were so stubborn I concluded they were bleach splatters from the upstairs balcony. Eventually I found a way of getting these marks off, by using a solution of 50:50 vinegar and water, or neat vinegar for the more stubborn stains. The cause of the staining remained a mystery.

I wondered what could have caused this problem because I wasn't doing anything differently that I did before I moved here. I had an idea that it might have been the water softening powder precipitating some kind of deposit. So I used the softening powder sparingly and I added some vinegar to the final rinse too and it seemed to keep the problem at bay.

Then I discovered the Liff Softest magnetic descaler fitted to the water supply.

What I am wondering is what does the magnetic decaler do to the water supply, in practical terms?

Does this mean that I don't need to use a water softening powder in the washing machine? Will it prevent the buildup of limescale in the washing machine? Could the treated water and the water softening powder cause some sort of chemical reaction that caused the white deposits?

I guess I just want to have stain-free clothes and prevent the buildup of limescale on the element and inside the machine.