Is This Mold On My Wall? Should I Be Concerned? What Should I Do?

Hello people of the forum!

I just noticed these spots that cover all the lenght of the top of one of my walls, and I'm concerned It might be mold.

Not sure If It's related at all with a problem I recently had with a moldy mattress (I threw It away, got a new one amd fixed the cause of moldinesss).

I speculate mold might have formed there because of humidity that is coming from my neighbors flat, as the spot where the mold is present is exactly under my ceiling (my neighbors floor).

I'm not sure how concerned should I be about this.
I cleaned it with water and some wet tissues to feel safer, will it come back?

Can someone please confirm if this is mold? What should I do? Should I be concerned about It's expansion and for my health?

Many thanks in advance!

https://imgur.com/Wi9BtD5

https://imgur.com/5vE39W5

https://imgur.com/Yu6zPhV