Equip the mindflayer ring as Celica and fight the normal black ones.
During the battle, there is a chance one of them will turn grey. Your base luck also increases how soon you encounter one.
Once you find it, defeat the rest. and lose to the grey one.
Grey cyclopus does not apply mind control it only does status effects of sleep and confusion I've done multiple encounters in the marsh and every time the grey one shows up that's what it does. I've read a walkthrough about a white variant similar to the one encountered when assisting Miriam with the Bee-Girl questline.