The Kryptonite Kid
In September 1960 the telepathic Kid and his Kryptonite Dog pass through a green cloud in space, both gaining an emerald hue and the attributes of Kryptonite, deadly to Superboy and all survivors of Krypton. The duo journey to Earth and use their Midas-like ability to transform everyday objects into Kryptonite, threatening the lives of Superboy and Krypto --who are only saved at the last minute by the 5th-Dimensional magic of Mr. Mxyzptlk (SB No. 83/1: "The Dreams of Doom").
Later, the villain also threatens the Man of Tomorrow as an adult (for example, S No. 299, May 1976: "The Double-or-Nothing Life of Superman!").
In a subsequent adventure, a second and different figure, known as the Kryptonite Man and a refugee from Krypton itself, seeks to destroy the Man of Steel (S No. 397, Jul 1984: "The Born-Again Kryptonite Man!").