That's a lot of weight for a pane of glass. Additionally, if it's tempered (which I suspect it must be), then when it fails, it'll fail catastrophically. It'll literally explode, sending glass bits everywhere. Now, it's actually not as dangerous as it sounds, the pieces of glass will be "dull" and won't cut to the bone, but can still cause nicks and scratches. You also risk breathing in glass dust. The reason why tempered glass can explode is because it's been cooled in such a way so that the exterior is putting the interior under pressure, and the interior is pushing out and giving the exterior strength. However, microfractures can form over time, especially when the glass flexes (like heavy weight being unsupported by a non-glass structure). As these microfractures occur, the pane gets weaker and weaker until the exterior can no longer keep the interior under pressure, at which point the entire thing explodes.

Tempered glass desks are pretty safe so long as you aren't putting too much weight on them, but a monitor mount, especially one that tall, is going to exert a ton of torque on the glass.

Glass cracks easily.