Part of macOS
MTLCompilerServiceGraphics compiler
○ Leave it
Prepares graphics code for games and apps.
What it is
Compiles graphics shaders. Busy briefly after installing or updating a game or app.
Why it might be busy
The first time a game or graphics app opens after an update. Usually a minute or two.
If you stop it
macOS starts it again straight away. Leave it.
The verdict
○ Leave it. Part of macOS, or something that starts again by itself. You never need to close these.
In Activity Monitor
MTLCompilerServiceMTLAssetUpgraderD
From Procession’s guide, in the words the app shows.