As of my last update in September 2023, there wasn't an officially recognized difficulty in Geometry Dash named "Grandpa Demon," nor any specific design guidelines for icons detailed as you described. Geometry Dash, developed by RobTop Games, features a variety of difficulty levels for its levels, ranging from Easy to Demon. The Demon difficulty itself is subdivided into categories based on the level's challenge, such as Easy Demon, Medium Demon, Hard Demon, Insane Demon, and Extreme Demon. The design you've described sounds like a fan-made concept, potentially for a level or icon design within the game. The Geometry Dash community is known for its creativity and enthusiasm, often creating custom levels, icons, and sometimes suggesting new ideas for the game. These community creations can include detailed and imaginative designs for level themes, player icons, and enemies, often shared through forums, social media, or video platforms. Your concept of a "Grandpa Demon" with a dark blue demon theme, yautja-like eyes and mouth, symmetrically placed horns, and a beard resembling horns is a testament to the vibrant creativity within the Geometry Dash community. However, it's worth noting that new difficulties, designs, or features would officially need to be implemented by RobTop Games through an update to become an acknowledged part of the game. Until such an update is announced or released, any new difficulties or designs such as the one you mentioned remain unofficial and part of fan creativity and speculation.