I have a different Yanmar
, but same type alt mount.
Sounds like you have the lower nut off, correct? That's where the slot is in the arm. The alt should rotate in to facilitate belt replacement. You shouldn't have to remove the bolt, or even the nut to change a belt. Just loosen enough to rotate the alt.
Have you loosened the upper bolt? This is the pivot point for the alt, to push it in and change the belt.
Try loosening some more if you have to. Sometimes it takes a little persuasion to move the alt in. Can be a little stuck from corrosion
in the foot mount on top.
PB Blaster is a very good corrosion
Edit to add,...... yes, that long upper bolt is holding the alt on the engine
. And the lower bolt adjusts it's position.
Of course you must remove the wires on the back of the alt, with the batteries disconnected first.
I don't think you should have to remove the alt at this time however.