トロ顔 (melting face)
A facial expression where the character appears to be in an ecstatic trance and their face almost seems to be melting in pleasure.
Example: post post #1164935
It is often sexual in nature, but not always. It may also be seen when a character is eating their favorite food, for example.
Not to be confused with ahegao, which is a more wild expression. Torogao indicates trance-like pleasure.