Commander X<II, his engineer, and his weapons officer. We're not sure which is which.
Several aliens appear over the course of the series, discounting the Biomass. Among them are two lawyers, an alien with a glowing screen for a head, Commander X<II, Commander X<II's engineer and weapons officer, and a smartbot. Generally when referring to the aliens, most readers will think of them as just 'Commander X<II and his crew', but for completionist's sake we shall be touching briefly on all of them.
Commander X<II (And his crew)
Commander X<II is the leader of the ill-fated attempt by the Invader's Union to colonize Earth after it was designated a 'dead planet'. After abducting Snippy it was noted by either Commander X<II or one of his crew that they had been unable to abduct Zee Captain, and so they sent out a smartbot to capture him.
Zee Captain disabled it with tea, murdered Commander X<II's weapons officer with cake, and exploded his engineer's head by way of an angry mask-less stare. (The only time that we have had any indication of Zee Captain removing his mask as yet) After that he stole the commander's pants to make a parachute for escaping and switched out Snippy for a bomb with Snippy's face drawn on the side.
Despite either Commander X<II's or his engineer's claims to be 'quite familiar with human anatomy' neither of them apparantly noted the difference until it was too late, although it's quite possible they were too traumatized to do anything. At some point, Commander X<II was able to escape via sub-atomic transporter, leaving him the only known survivor of the mission. Thanks to his ship, crew, and pants being unaccounted for, he was held on trial by the Galactic Tribunal. It's unknown what his eventual fate was. However, his status as 'invader' seems to have been revoked.
Commander X<II (and his crew) are all humanoid in form, although it's unknown what their heads look like under their helmets. They appear to have greyish skin, judging from Commander X<II's brief pants-less scene.
The Alien with the Glowing Screen Head
In episodes 24 and 28, we meet an alien with a glowing screen for a head. In episode 24 we see him reviewing the case of Commander X<II's mission. In episode 28 he's moved on to broadcasting the news of the universe's impending doom and the Invader's Union filing a lawsuit against Earth. Either our glowing screen man is a bit of renaissance man, or there are simply multiple glowing screen headed aliens running around.
Much like Commander X<II, the glowing screen man is humanoid in appearance all but for his head, which you can probably guess as to why it's noteworthy, and his hands which have extremely long fingers. His skin appears to have the same greyish tone as Commander X<II's species. He dresses something like a scientist, but with a tie that looks like some sort of glowing ball thing. At one point, with the help of a brainscanner, the glowing screen man uses his face to show video footage of Commander X<II's memories of Zee Captain's attack.
The Lawyers
Two lawyers appear in a single panel in Episode 24. Both of them are humanoid except for their heads, one of which looks like a squid and the other of which looks like a shrimp. The shrimp lawyer appears to be Commander X<II's defence lawyer, although odds are he isn't very good at his job as the squid-headed lawyer chides him for repeatedly using 'contemporary insanity' as a defense.