Pitch doesn't reach home plate
 

R1 with pitcher from the stretch. Ball slips while pitcher is delivering to the plate. Ball lands in grass between home and mound. Ball rolls to a stop before reaching home plate. PU calls balk and advances runner. Balk was called for ball not reaching home plate and not for any other action by pitcher.

Coaches challenged it as a ball and not a balk as it didn't pass through strike zone or cross a foul line.

I found discussion where pitcher drops the ball on the mound or throws it into foul territory but nothing about not getting to home plate.

Is balk the correct call?

Yes. If the ball crosses the foul line, it's a ball. If it doesn't, it's a balk (with runners on base; with no runners this last action is "nothing.")

Ironic, considering it's the not crossing of the foul line that makes it a balk and not a ball.

As always, thanks for the information.