No, it's not saying the enemy is already effected by the spell, it's saying "was not put under the spell". The spell is the subject actually so it's saying the spell was ineffective. In this case however, the spell was ineffective because the enemy is already affected by the spell so there is no reason not to just say so if you think it would be clearer.

How about?: "Spell had no effect."

I am intrigued by this idea.
Does losing EXP cause you to level-down?
Do you get it back after defeating enemies, or is like other games where enemies try to steal your money/items and then try to run away so you can't get them back.

What I have noted is this: The Incubus steals 2000 EXP when it uses that spell. Then, on the next round or so, I strike it down and gain 2000 EXP for everyone still standing in the active party. Therefore, it's a zero net gain or loss in that instance. But hot-switching parties to allow another group to cherry-pick that 2000 EXP would make the loss stick for the vacating fighter who was affected. We would need to experiment to see if someone could level down. At that time in the game, the average fighter needs about 8,000 to 10,000 EXP to level up, so the math would need to work also. Finally, the Incubus is rare, so experimentation would be difficult. I've only found it guarding a handful of special treasures in one of the four final towers.

Ah looks like I better jot this quirkness down in my checklist so I can run some late game tests and ascertain the effects.
Either way it would be detrimental for the player since even if the levels don't drop, making the battle drag too long
would mean having to basically regain the exp needed to advance to the next levels.

Sounds like a plan ^.^
I'll check my email box regularly then.