What do you do when approaching a green marker bouy going upstream?

Keep the marker to your port (left) side. Heading down stream it'll be to starboard.

Usually there will be a red marker to the starboard headed upstream, and you should keep between the buoys , sometimes there will be midstream buoys, or even preferred channel markers, marking safe deep water channels. They will be red over green, triangular, lettered or even numbered for starboard markers, and green over red, square, lettered or odd numbered for port side markers. Center channel markers may be yellow or white, hazard markers are black over white, or black over red. Inter-coastal waterway markers are yellow.

States have their own marker systems, heading up current though in all systems will be green to port, red to starboard and black marking hazards.

Lighted markers follow the same rules, green to port, red to starboard and white for hazards heading in from the sea/ocean. If the lights are moving they're on a vessel, stopped or anchored vessel should only show a white light or lights. Yellow lights indicate a vessel under tow.