Black spot in the backwash filter

Hi,
I noticed a black spot in our backwash filter that wasn’t there initially.
It cannot be removed by backwashing.
What could it be? Could it be harmful to health?

that is the filter insert,
I would change it now and observe if it comes back.
PS: maybe a lost sock will be found :eek:

Can you really replace it? I thought with a backwash filter you could only backwash?

No, of course you can change it. It is even recommended. I also replaced ours after 10 years. It no longer worked properly.