During June's first encounter with the Tenth Doctor, the Doctor was held captive by a group of aliens.
June described the aliens as "grey and blobby", and had skin that seemed to glisten with rainbow patterns that she likened to a puddle of oil. They were roughly a metre and a half in height. (PROSE: The Slitheen Excursion)
During their brief meeting, a group of these aliens were planning to use explosives to destroy Athens. In his attempt to stop them, the Tenth Doctor was held captive by one of them, called Bartholomew. He warned them that once human authorities had caught them, they would no doubt be dissected. The aliens were scared away by June, due to them being afraid of humans. (PROSE: The Slitheen Excursion)
June witnessed, in a conversation between the Tenth Doctor and Bartholomew, that they spoke a language that consisted of "clicks and whistles". (PROSE: The Slitheen Excursion)