Jellyhead-head is one of the Cannonballs. She sits slumped against a tree, nearby the other Cannonballs; Cannonballhead, Fishmaskhead, and Puddinghead.
The message in a bottle to locate this castaway was last visible 2 days ago on 4 July. It will be visible again in 4 days on 10 July. (wrong?) Once a message in a bottle is found and when a player has walked near its spawn location when it was visible it will always be visible, unless the message in the bottle or its treasure map is in the player's bank or inventory.
She is wearing a jellyhead. She appears to have died, having stopped moving and talking after putting the jellyhead on, meaning it might well have been an actual jelly.