EDIT: I believe I found the answer, it's something turned by this: https://ctatools.com/products/5065

Found some trash on the street that i'd like to take apart, but this screw is in my way.

It's like a hex bolt, but with 5 sides, and rounded (a bit like flower petals).

Here's another picture from the top:

I tried turning it with various tools (hard to get pliars in unfortunately), no success so far. The material is very soft, which doesn't help..