Once Koltin finishes eating, "The Hunt of Bubbul Gems" "The Search for Koltin" will immediately start.

Source: https://screenrant.com/zelda-totk-bubbul-gems-bubbulfrog-locations/

Speaking to both Koltin and Kilton will start the quest "The Hunt for Bubbul Gems."

Source: https://screenrant.com/zelda-totk-bubbul-gems-bubbulfrog-locations/

These are Bubbul Frog locations, so Link can use these light beams as guides to track down the remaining Bubbul Frogs.

Source: https://screenrant.com/totk-koltin-locations-zelda-tears-kingdom/

These are Bubbul Frog locations, so Link can use these light beams as guides to track down the remaining Bubbul Frogs.

Source: https://screenrant.com/totk-koltin-locations-zelda-tears-kingdom/

While they do not drop Rupees, they do produce the similarly-crystalline Bubbul gems, which can then be traded to Koltin in exchange for various rewards (including the Mystic armor set, which notably causes Link to lose Rupees instead of hearts).

Source: https://screenrant.com/zelda-tears-kingdom-blupees-rupees-caves-tok/