I remember reading part of a manhwa that was historical fantasy, likely Korean and full color, about an emperor or other high ranking guy that found and brought in the FL who is a mermaid.

I don’t think it was Die Melusine or Admiral Monster Wife. I remember a scene where some other girl was like “I found a mermaid” and the ML got nervous that the FL had been captured.

If you know the name, please help me find it! I don’t think the title had the word mermaid in it but I could be wrong

It doesn't sound like it, but just in case, maybe To Take a Mermaid's Heart?

Or maybe Mermaid Princess? (I don't know the story of this one at all, I'm just going off of the description)

You could also try looking through the categories Merfolk Female Lead and Mermaid/s to see if it's there.

Mermaid princess looks like a close guess but I don’t think that’s it. Thanks for the places to look though!