When the Alien Rangers left earth after successfully protecting it, that seemed like a final farewell. Luckily though, they have encountered the Power Rangers thrice since then.
The first occurrence was a distressing one. Aquitar was being contaminated big time by the Hydro Contaminators. It was something which even the Rangers couldn't handle. So their solution was sending Cestro to earth in a small spacecraft. Once there, his intentions were very clear. He had come for Billy and the knowledge which he possessed. A short time after, both Billy and Cestro went to Aquitar so they could pool their thoughts and solve the problems together. A few days later, Billy returned, and his mission was very successful. Aquitar was once again a safe planet to be on. There was a little more to this though. While there, Billy had received information regarding the creation of a new Zord. With this, he was later able to create Tommy's own Red BattleZord.
The second encounter effectively ended the search for the identity of the Gold Ranger. The Gold Ranger's Zord, Pyramidas, was shot down by bounty hunters in the deepest waters of Aquitar. The Alien Rangers found him, and tried to bring him to safety, but he could not survive in the watery environment. So the Rangers contacted Zordon in the Power Chamber, and shortly after, Trey, the Gold Ranger, was teleported safely to earth. The Aquitians had saved his powers..... and also his life.
The most recent meeting of the Power came in trying times. Billy was growing old. Very old. While working in the Power Chamber, side effects from the old growth regenerator he had used on himself took action, and he quickly turned into an old man. Zordon and Alpha did not know what to do to save him, so they once again called upon the Ranger's of Aquitar. Two of the Alien Rangers, Delphine and Cestro, came to help cure Billy's accelerated aging. They tried a gun, which was a remodification of the growth machine that Billy built during the time of the Orb of Doom, but it malfunctioned and in the process damaged the Power Chamber's teleportation system. They rebuilt it and tried it again, but once more, it failed. Cestria, an Aquitian healer, arrived equipped with Aquitian water for Billy, which was described as similar to the water from the Fountain of Youth, but that did not work either. Billy and Cestria exchanged some words that denoted that they were old friends, and with his trust in her, they took the most extreme measure. She took Billy back to Aquitar for long-term treatment. Later, after the Alien Rangers assisted the Power Rangers in fighting a tough battle, Cestria called over the viewing globe from Aquitar saying that Billy was restored to his normal self again. Once more, the Aquitians had proven to be useful allies in the fight for good, and had saved the life of the former Blue Ranger.