Can't edit a layer with a little square icon at the bottom right
I need to edit one layer of a file that I created six years ago and I can't find any way to do that. It was for something I have never done again since then.
Four of the layers, one of them being the one I need to edit, show a little icon on the layer in the sidebar. The layer is not editable.
It's a square which has, at each of the four angles, another very tiny square. I don't remember what I did at that time and have no idea what this icon means.

Can anyone help?
