Rainbow Goop is a recurring item within Hanazuki: Full of Treasures. It is a substance found abundantly in Hanazuki's moon.
Goop is a sticky hard substance that has served many uses in the show, given its versatility. From Goop fountains they are relatively fluid, but they easily harden to a glue-like substance once taken out. When left to dry, it turns hard and brittle. According to Chicken Plant in What's a Chicken Plant?, Goop is bitter when eaten.
Goop is most helpful in the show because it is the source of nourishment of Treasure Trees when the substance is slathered on their trunks, which keeps them strong and healthy. But other than that it has also served as: glue for Kiazuki to fix her spacesurfer, Hanazuki to save Yellow from Chicken Plant given its bitterness, and for the Hemkas to make a bouncy sheet of blanket.