It may not actually be missing, a character may be using a different version of it than what your files in the Mods folder have. BepInEx can't distinguish between a mod that is actually missing and a mod which has a different version on a character than in your game files. Try replacing all accessories on whichever character triggers the message until it stops.
If you replace all accessories and still get the message, try the Initsetting.exe updater.
It's a hair mod. when i load a card, the hair is loaded differently from the image in the actual card. i tried changing the hair to one of the default hairstyles and the message dissapeared. heres the card for ref. I asked the sharer about the hair mod but he/she is not replying.