This item is exists? You can grow it, anyone know?

Not available in-game. It is in the game files hence why it shows in all regions but has never been implemented like a lot of other items in the database but not used for whatever reason.

These would be very OP given the rarity of truffles via gathering. They can be harvested from Polly Forest NPCs so it is an other option for them now at least.

Simple Alchemy on an unknown seed and a truffle mushroom.

Unknown seed is a rare byproduct of alchemy. Master 11 and have only ever gotten 3.

This item does not exist
There is no High-quality Truffle Mushroom ingame

The correct hypha = Truffle Mushroom Crop