Possessive/obsessive semes: (can't guarantee about green flagish for all but yeaa)
Roses and Champagne
https://www.mangago.zone/read-manga/roses_and_champagne/
Waterside Night
https://www.mangago.zone/read-manga/night_by_the_sea/
Dreadful Night
https://www.mangago.zone/read-manga/dreadful_night/
Dead man Switch
https://www.mangago.zone/read-manga/dead_man_switch/
A Link Between Relationships
https://www.mangago.zone/read-manga/a_link_between_relationships/
Park Hanhoo's Manager
https://www.mangago.zone/read-manga/park_hanhoo_s_manager/
Mad Dog
https://www.mangago.zone/read-manga/mad_dog/
After Killing You
https://www.mangago.zone/read-manga/after_killing_you/
Monday'saviour
https://www.mangago.zone/read-manga/monday_saviour/
Love Plan
https://www.mangago.zone/read-manga/love_plan/
enjoy (｡•ᴗ-)✧
Looking for manhwa with obsessive or yandere like male lead (and sort of green flag-ish) can be straight or yaoi :))