Over the next eight issues, Ultra returns home to his girlfriend (Bonnie Blake), and fights Dr. Taxo and the Moon Raiders (in Mystery In Space #104), Dr. Dynamo and his energy creatures (Mystery In Space #105), Craniac and Tragg (Mystery In Space #106), splits apart and has to reassemble (Mystery In Space #107), fights the giants Kratum and Vaaro (Mystery In Space #108), faces Venusian Jal-Rel and Martians Klu-Kann, Tojeel, and Arrkon (and gains the ability to turn back into Ace Arn...all in the only issue that doesn't have him on the cover, Mystery In Space #109), and with his final regular appearance in Mystery In Space #110 (September, 1966) faces off against the Pied Piper of Pluto! Then, Mystery in Space was...gone! (Cancelled, not much of a mystery, sad to say).
Ultra next appears during the Infinite Crisis (being involved with Donna Troy and the space faring heroes involved in the Rann-Thanagar War), and in Superman/Batman #31-33 (as part of a collection of outer space heroes).
...and must have liked the character, as he both wrote and drew the story of "The Life And Death of Ace Arn" which had 8 pages of Ultra the Multi-Alien all on his own in a Vertigo comic!
The New 52
...as he included the character (with a slightly different origin, that of being a creation of the alien shape-shifter, Byth), in the early issues of Justice League United, including issue #2, entitled "Mystery In Space"!.
Not bad for a nearly forgotten character of the mid-1960s?
Silver Age Sci-Fi Companion of 2007, Ultra didn't make the interior (still, it's an incredible collection of facts on folks like those he replaced in Mystery in Space like Adam Strange and Space Ranger, as well as the Star Rovers, Captain Comet, the Space Cabby and the Atomic Knights...and more!).
Ultra did make it into the American Comic Book Chronicles:1965-1969 by John Wells and Keith Dallas, wherein they said this of Ultra.."The concept itself wasn't necessarily bad...but Ultra just looked silly. He was part bald blue man and part hairy green monster set atop a bird's leg and a lightning bolt."....and that was one of the joys of Silver Age science fiction!