Probably trying to cast a spell ID that doesn't exist or something, or maybe it's attempting to cast Pearl although that doesn't cost 180MP. Thanks for testing the greens as well.
That was my first guess, yeah. It reminds me of casting Berserk on certain enemies in vanilla (where they have no valid Berserk command and so game expects them to have some absurd amount of MP - 65,535, maybe? - and actually throws up an error message if you do hack them to have sufficient MP to attempt casting it, heheh, good times!) - but I didn't want to assume, I'm an expert by no means, I just like to experiment, heh. Still. Hopefully it'll be easy enough to fix. Also Glutton did eventually succeed in casting something on me - Lagomorph, I believe it was?, though it still missed - but that was after several encounters (seeing if he'd do anything different) and him off and on randomly spouting "MP depleted" at me.
And you're welcome, no problem on testing those Greens - I actually did that before I even came back and saw your answer (what luck), just to see if it was some kind of bad luck on my part or if they were just being finicky (it would match their canon personalities in some other FFs, heh!) or if they just weren't gonna cast it at all no matter what I did, so. But I'm certainly glad to help even in small ways.
Also my game just crashed...or at least froze anyway, damn. I beat the red palette swap of the giant spider in the Cavern of the Gi (Red Stinger, that's right) - successfully Morphing it into an Elixir - my controls stopped working immediately as soon as it faded back into the field map. I tabbed in and out of the window, unplugged and replugged my controller, even tried using keyboard controls to see if it was just my controller acting up or something. Nothing worked. After trying several things, the game stopped showing any image at all, stuck with with a greyed out version of the Steam overlay (which I can't dismiss either, it just grays out and fades back in if I hit Shift+Tab, it won't actually fully leave and return me to the game), with the music even still playing. Hell. That's what I get for not saving immediately after doing a Fort Condor mission again, I broke the cardinal rule of jRPGs if not video games in general, save often, heh. Welp. I guess things happen. Hopefully it won't crash or screw up on me again - if it does I'll let you know. I'm presuming/hoping it was a one-off of Windows just being Windows - but I guess I shouldn't assume.
Edit: Crashed again similarly on my return to Fort Condor - controls locked after the last enemy died in the minigame, and the game never acknowledged my win. But on the third attempt, everything worked out fine - I'm currently at the save point right before the boss of the cavern.
Likely unrelated, but, for some reason, the graphics on Quake3 will sometimes/often completely flip out when you cast them and a Gi Spectre is in battle. (You'd have no reason to cast it on the Spectre itself, of course, being floating and thus immune to Earth damage - but the other enemies in its encounter don't have Earth immunity, so.) By which I mean, the model starts grabbing graphics from random stuff on the screen (including the HUD) and ends up becoming a multi-color mess of garbage blocks (though they go away as soon as the animation ends). Granted, I can't imagine this would be a real big priority, assuming it even is something to do with your mod - I highly doubt this would be a common scenario, heh, someone actually bothering to get the AP necessary to obtain Quake3 by this point. (Unless Gi Spectre can be fought elsewhere - like the Battle Square, at some point?) But I wanted to mention it anyway, just in case.