The skull-like face set in his helmet was one of ornament only, and not Rum's true face, so he could have been a representative of any of the thousands of humanoid species in the galaxy.[1]
Rum Sleg's costume was present in the crowd of the Boonta Eve Classic. It stands to reason this was Rum. Rum Sleg was named for his Young Jedi CCG card after Dan Bojanowski's Star Wars RPG character.
Rum Sleg after death. In the comic, Survivors, an individual similar in appearance to Rum Sleg (possibly Sleg himself) was killed by Uda-Khalid, and was visible among the crowd of Uda-Khalid's victims.[2]
Rum in the crowd.An action figure of Rum Sleg was made for Hasbro's 2009 Legacy Collection. The figure featured a removable helmet, which did not reveal Sleg's face behind his faceplate but does show that he has whitish, lumpy, slimy skin and a brace of dreadlock-like appendages that are tied back in a ponytail. Whatever species Sleg may be, it is therefore confirmed that he is not human.
In the Tales comic Best Birthday Ever, Rum Sleg is visible standing in line at the Mos Eisley equivalent of a Post Office. The comic is non-canon, however.