What does a Green Celestial Flask do? x3 I feel so weird asking this, hehe.

& also, Happy Luminaire!

SubetaHQ says it's simply a toy. Using it to play with your pet would cause it to disappear (like all toys) but you would receive the R99 Toy Achievement (for using a R99 to play with your pet) because it is a R99 item. :3

I was wondering that too, it has no use option

I have one and it doesn't have the option to play with a pet. It really has no use that I can see.