The black bottom is some sort of rubber bag, connected to the gray rubber tube which is normally inside in the west-south direction (also bag like; very flexible), and the water goes out from the tube.
It's been working well for years but today when I was dumping very finely-cut tea leaves, like the kind inside tea bags, it suddenly failed and water couldn't be drained, but there was absolutely nothing greasy. After messing with the tube for a hour or so it's back to normal, and I can't even tell what I did.
Can I just remove the stupid bag or dig holes on it? It's not a trap like those in shower or kitchen, and the design seems to keep some water to prevent undesirables from going up, but quite annoying given there is no manual or instructions left.
PS: it's not even a part of the drain cover, but maybe something old that wasn't removed during pipe renovations. I can't find anything similar to it in Finnish shops.