am I not getting something about how eating seedlings makes one stupid?
Whoops, it was ミョウガ and not ミョウが, so it's not seedlings, it's Japanese ginger (a bud of which Matsuwa is holding in the 3rd panel). And going by a quick googling, it's an old saying that 'over-eating myoga makes you forget things', leading into 'become dumb'; this seems to stem from that thought that ginger is nice and healthy in small amounts, but isn't all that great for you in large amounts.
So I guess there's something in the (sea) water that makes everybody in the vicinity of Nonco's base utter loonies?
Abyssal microplastics
Hah?CLEVER!!Wha-...!?Show humanity, even to one's enemy.If you wanna eat, then you're gonna have to work with us, see!!What's this!? You're sure full of piss and vinegar, aren't you!!So... so terrifying...!ThudWe make a whole bunch of tasty myouga, send it over, and let them eat up.And when they eat too much myouga,I've thought of a good idea!Matsuwa's speech is written in katakana like the abyssals' is - perhaps she is emulating her captors.they'll get stupid!An old Japanese superstition. Google "Cuuda-pantaka myoga".You no-drop bitches!!What are you doing?This little girl, she's... she's...Okay!ThudThud