Responses
I don't remember what the color of the mermaid's hair is but everything else matches.
https://www.mangago.zone/read-manga/new_world_lovetopia/
This one has a red hair mermaid but the story is completely different.
https://www.mangago.zone/read-manga/wadatsumi_no_yometori/
Thought it may have been color but seems I was talking about something else but this is it I’ve been trying to find this forever thank you!! (づ￣ ³￣)づ
I remember reading some random bl where the main character took over maybe the place from his grandpa and it was like an indoor place where it had a merman with red hair and I remember it also had an angel at the beginning of the story and the love interest(I can’t remember) was a secret demon maybe and I think he had a light tan skin I can’t remember since it’s been a long time but please me help find it I was looking for a long time ┗( T﹏T )┛