Although the aliens are never entirely seen, some among them are obviously of rather considerable size, with claws the size of Spiff. When Spiff mentions Scum Beings, he will usually accompany their name with epithets such as "hideous", "disgusting" or "dreaded".
Their ships are not in proportion to the creatures' size, meaning the sizes of scum beings probably vary. Scum being ships have become the basis for most alien ships in the Spaceman Spiff universe.
Once, Spiff is being chased by a ship much akin to scum being craft, and the occupants are seen. They resemble large piles of goo wearing helmets and goggles, and speak an indecipherable language.
They appeared in a weekday strip, where Spiff was being chased by one. He attempted to climb into his ship via a ladder, but it grabbed him. In this instance, the scum beings were very large and were analogous to Miss Wormwood making Calvin go inside because recess was over.
The scum beings returned in a Sunday strip where they pursued Spiff using one of their ships. Spiff stated they were from Planet Q-13, and they were seen using Frap Ray as a weapon. The scum beings shot down Spiff, and he crashed on a planet. This was analogous to Calvin going down a slide.
They possibly returned in another Sunday strip. These possible scum beings pursued Spiff in their ship and fired Explode-O-Ray at him. They missed, and Spiff flew down to a planet, but the aliens kept chasing him and Spiff's attempt to make them crash by flying his ship through an arch failed due to the scum being ship's maneuverability. This was analogous to Calvin's mom trying to catch Calvin to make him go to bed.