The interesting thing is that Devlin's appearance (he'd get stuck in that time as Bauble Boy for three more Legion holiday specials) isn't even the only cross over in that panel.
On the right, Year Allon is, if not under cover, then certainly under foliage.
On the right above is her later, similar, transformation in order to investigate Earth Man. But what is her mission here? Is she investigating one of the Legionnaires?
Eagle eyed readers may spot that the sun has turned green in the above panel. It's yet another lost Legion tale, where the Legion think that a Kryptonite Cloud has covered the sun. Later they find out it's Chlorophyll Kid trapped by a mad time travelling Swamp Thing (appearing as the tree outside the room in the above panel), using The Green to prevent the return of a blood thirsty Animal Man from the Red.
It's Yera who saves the day along with Colour Kid, who shows us that green and red are just colours in the wider spectrum that we all belong to.
It might have been a holiday special, but Levitz and Garcia Lopez didn't shirk from providing Easter eggs for the regular readers.