Looks like some screen burns in. Was it sitting with the picture on the screen for a very long time? AMOLED tends to let some burn in once it has been left for a long time. Just from some quick googling, it sounds like it shouldn't be permanent.
I know there are videos that just flash around many colors to help accelerate the return to the normality of plasma screens, so maybe you can just jump in and out of programs on your watch and make the screen change a lot, which can help ? Id say just give it a little and if it's still there for 2 days then take it to get it to see it.