The Pearl Radar is the radar that Hippo uses to locate the pearls of the Mermaid Princesses.
Hippo, in the first few episodes of the first season, used the Pearl Radar to find Lucia's pearl after she gave it to Kaito to save his life. This pearl is needed for the coming of age ceremony which already was delayed by a year for Lucia. The pearl is found in the very first episode, though. Hippo also used the radar to find the rest of the Mermaid Princesses to help summon Aqua Regina, and it was a plot device for several more episodes. The pearl radar never appeared in the second season.
The Pearl Radar is a rectangular, pink radar with a red led on the top of it. It is as big and long as Hippo's penguin-form hand. It has a green screen with a dark green grid, in which white spots appear when pearls are near to the radar.