That Time I Got Reincarnated as a Slime: Tensei shitara Slime datta Ken

[Ryllharu]

Also, I thought Clayman was already a Demon Lord. Is he trying to become a...double Demon Lord?

Great Sage kind of explained this the previous episode. Rimuru qualified as a Demon Lord Seed after consuming the Orc Lord, who was also being prepped into becoming a Demon Lord Seed.

Anyone strong enough or with territory can call themselves a demon lord, the weak douchebag in the plague mask messing with the Orc Lord called himself one, right before he got eaten.

But whatever this does is different somehow. A "true Demon Lord."

Presumably, Clayman is a Demon Lord Seed already as well, or maybe just below. Either way, I guess he somehow hasn't killed enough personally, or killed enough at once. He's a manipulator, making others like Myulan do his dirty work. Rimuru always faces threats head on. Maybe that's the difference.