I'm usually pretty good at picking out the books from descriptions, but this is one I read that I just can't figure out, so I figure I should try the brain power here.
Rememebered details: YA, possibly Juvenille sci-fi book 15+ years old. Main characters were a boy and a girl living in a controlled environment. Dome? They explore their surroundings and discover alien technology--including something that transforms the boy into one of these aliens. Alien description: biological, tentacled--with tentacles having odd shaped appendages, single eye which circled the entire head giving them a 360 view.
Other bits I remember aren't useful to identification, I don't think, but I can try to fill in some more details if it helps.
EDIT: Updating with new information that might help identify as I've recalled more in seeing what is not quite right about the proposed answer so far.
The recollection of the dome, or otherwise controlled environment, comes from a passage indicating that the weather is controlled; it rains for a brief period each day, but the duration of precipitation is random. The two protagonists are bicycling at the start of the story when the rain occurs.
The transformation occurs somehow, as noted above, causing the child to become one of these alien creatures, who they aren't familiar with. The aliens are gone or dead (I seem to recall they come back at the very end--but how much is true memory is hard to say). He uses his appendages to operate the alien machinery to do something. The description I remember, but which might be wrong--so hopefully I'm not sending anyone on a goose chase--was that the tentacle had a triangular tip, which fit into receptors allowing the machinery to be operated.
