Weird bruise thing behind knee

[Selkie]

Could be a blown capillary. I get them (Irish skin--fair). Usually they aren't that big, though. Just red dots, like moles. They aren't anything to worry about, and my doctor told me that as I get older, I'd get more of them. A prediction that is coming true, unfortunately. I did have a larger one on my thigh a few years ago and went to the derm because I thought it might be a melanoma (uneven edges, red in color, etc). She said it was just another blown capillary. Weirdly enough, it went away shortly thereafter.

If I were you, I'd get it looked over by a derm. I can't tell too much from your photo---it might be a mole.