Thank you so much for your ideas. In the meantime, I asked my dad and he said it looks like a soldering flux, so now I'm not worried anymore.
It is Atari 800 XL (I thought I mentioned it in my first post but apparently I forgot, sorry
). This machine was quite popular in Poland in the late 80s and early 90s. Games were easily available on cassettes and very cheap (between 1989 and 1994 there was no copyright law in Poland as we were in the transition between a communist and a free-market economy), and there was a vibrant demo scene and even a few game publishers like LK Avalon.
I got it in 1994 from my uncle as they bought Amiga and it wasn't fun for them anymore. I had no games unfortunately as it was already 10 years old and cassettes were lost. But I remember I wrote some simple BASIC programs on it and I think it had some impact on me as today I am a software developer.