Killing the mouboo (or injuring the sick tree - this is a spoiler, right?) will make you lose all nature magic forever as there is nothing you can do to make things all right with Sagatha again (there is a nice "natural armor spell" you might not want to miss and also who knows what other future spells of higher magic lvl). I didn't get the black turtle neck either.
Giving the mouboo a large healing potion instantly heals it without any hugs, kisses or spells, but the turtleneck is lost forever. How about creating a dummy new character to kill that mouboo and trade the turtleneck to you if you feel you really need it? You can delete that character afterwards. I know, I'm evil-thinking and all pink cute little girls will disrespect me forever.

I feel that black turtle neck's prices will suddenly drop after this post... And it makes me feel happy.
Later edit: Actually, a lvl 1 is too weak to kill the mouboo, sick as it is, so the devs thought about this (congrats!) I'm working on it though...