How do you say grumpy in Japanese?

Answer 1

You can use 気難しい /ki mu zu ka shi/, for 'to be grumpy' you can use expressions such as 顔を顰める /ka o wo shi ka me ru/ or 不機嫌な顔をする /fu ki gen na ka o wo su ru/.

Answer 2

Okorippoi, or in Japanese Katakana, 気難しい.

Hope I helped.