Error with Serial Communication between Processing and Arduino
I know nothing about "processing", but just based on looking at your code, it seems that background(0) clears the graphics frame, and ellipse(...) draws the circle. You are doing the former unconditionally on every iteration through draw(), but the latter only when you get a message. So it isn't surprising that you get a lot of flickering.

The fix would be to move the background(0) call to immediately before the ellipse(...) call.