Yes, this is another Isekai. No it's not unique whatsoever, it's just a power fantasy isekai with the protagonist reincarnated as a slime instead of a hero. Yet the best thing about Tensei Slime is that it's not trying to be different, but it's trying to improve the Isekai genre to the fullest with its detailed world building. The side characters are equally as important as the main ones and there are a lot of interactions between them. The focus is more about building a civilization than just battles, even the wars are for building or defending the country. Rimuru the protagonist is absolutely charming and adorable, but it's a shame that it has no gender.

I recommend reading the Light Novel too for more detailed world building.