ref donburke
and then
Friable comes to mind, then again friable and musty don't seem to hang well together.
The grain has undescribed flavours (which may include musty)
but as to the beer
Is the grain musty or does it appear contaminated?
If it is musty would you expect, after mashing and boiling that the mustiness would be transferred (I would but thats beside the point)
If its not musty (and as yet has not been described so) then why else would your beer be musty, you do not have infections, you sanitise, every othe beer you make is top class?
No need to thank me or others for their input, you clearly nailed it "its the grain"
K