I lived in Hawai'i for three years and remember blue stop signs, but not green. Green road signs in Hawai'i would fail to stand out for 3/4 of the year and defeat their purpose.
It could possibly be a stop sign from Ko Olina on O'ahu, but I never spent enough time there to actually absorb the color of their road signs.
Also, for those wondering about the inevitable followup question, a stop sign has to be placed by a local or state government to be enforceable, and is required to follow the exact standard specifications. Tickets have been thrown out due to stop signs being a slightly wrong shade of red, so blue or green are definitely out.
If it's private property it could be anything. Maybe it's a golf course or something plant related. Maybe it's somewhere plant or tree related. Maybe it's just camouflage to keep the visual noise down.