What is this thing and how do I delete it?

Hi,

I was busy routing tracks on a board and I did something - I’m not sure what - pressed some sort of combination of keys. Anyway, Kicad put this cross-hair ‘target’ on my board. I don’t know what it is, and I can’t seem to be able to delete it. It stays the same size when I zoom in and out, and is still there if I turn all the visible layers off.

Can anybody tell me what it is and how I can delete it?

Morph.