Rin, I’m pretty sure it’s saying “Multi” as in the purple key is a balanced mix of Power and Speed/Sky, being a purple key which is a mix of Red and Blue, of course I could be wrong, but it’s most likely saying “Multi” not “Mulch”
I dunno... The guy seems to love his plants. Seems oddly fitting in a joke kinda way. And I think the reason Rin called it Mulch is cuz it's probably spelled like "Mul-Chi" despite the way it's pronounced.