ᐅ Black spot in the backwash filter

Hi,
I noticed a black spot in our backwash filter that wasn’t there at the beginning.
It can’t be removed by backflushing.
What could it be? Could it be harmful to health?

That is the filter cartridge. I would replace it now and monitor if it happens again.
PS: maybe a lost sock will turn up 😱

Is it really possible to replace it?
I thought with a backwash filter you could only backwash?

No, of course you can replace it. In fact, it is even recommended. I also replaced ours after 10 years. It was no longer functioning properly.