Andromedans are one of several species native to the Andromeda Galaxy. This species in particular is capable of altering its shape, though with considerable effort -- similar to the Kelvans to whom the Andromedans may have been related. Several of them were able to infiltrate the Star One facility by posing as humans. In their true form, they have no discernible appendages and a faint green glow. This form has only been seen upon their deaths, as contact with the Andromedans has been extremely limited.
The Andromedans, through their ruse, were able to destroy the galactic boundary maintained by the Terran Federation and invade the Milky Way. The ensuing Intergalactic War was only narrowly won by the Federation. It presumed that the motivation for this invasion was the same as that of the Kelvan Empire some centuries earlier -- as radiation levels in the Andromeda Galaxy rose to intolerable levels, the inhabitants sought other galaxies to make their home in.
The Andromedans are derived from Blake's 7.