I am friends that are a pair of twins and this has actually happened to both of them on separate occasions. Each came up with a cool idea and did all the thinking to create a story about it only to find out that the central premise was the same as some TV show/movie that had come out somewhat recently and was relatively popular. It's a real downer. I guess the silver lining is that at least your idea was cool enough that people liked it?
Honestly though, nothing is really original these days. A lot of works out there are all based on the same premise. Just look at the amount of isekai stories we have around right now. I'd say the premise of "eye color indicating amount of magic power" is no more specific of a premise then "Gamer reincarnating into a game-like fantasy world by a king wanting them to defeat the demon king" and there is a ton of those floating around and people still read them.
As long as you go a different direction with your premise then this story's main plot line it'll be just as original as most stories out there. Telling stories are less about being original and more about taking the typical tropes for that genre and twisting them in a way to make something new.
I've had a dream about a year and a half ago. It takes place in a world where the more amber ones eyes are, the more magic power they hold. I don't feel like I can continue to further that project. This is the first time I've heard of this manga. I'm frustrated. Oml. I don't know what I should do. I thought is was so original tooooo...