The Yellow Light Aliens, appeared in the episode "The Tale of the Unexpected Visitor", it's unknown who portrayed them.
Perch and his friend Jeff were messing around with Jeff's dad computer. Jeff's dad was a brilliant scientist who
invented a program called the Peabody Project. This program was designed to send messages and communicate with beings and lifeforms from other planets. However, Jeff's Dad never tried it yet and wasn't sure it would even work.
When Jeff was checking on his little brother, Perch's curiosity got the best of him. He typed in random co-ordinance 456610237 to a random planet. Then thinking it was all a game he sends a high note musical message that was instantly sent through satellites into Outer Space.
Perch then got a reply saying the message was received; much to his disbelief. Perch tried to shut the computers down, but suddenly the computers were all blaring off with a siren alarm saying there is an incoming message. Perch doesn't believe it and shuts the computers down. Then suddenly they all came back on and a full blast message is heard in the form of a low note musical message.
This reply came from the young yellow light alien child. It quickly traveled to earth in what appeared to be an instant teleport. Mostly it would only make the same low note musical message message it first replied to Perch with. Then it set up a glowing yellow light spider web and captured Jeff's dog Montana.
Then the alien child set up more spider web traps, and one by one it captured Perch and Jeff's little brother Bobby. Jeff was able to free them from the capsules they were sealed in by playing the same high musical note.
Then soon the Mother Alien came to bring her child home. She proved to the Jeff, Perch, Bobby that their race was highly intelligent. She tried every language until she figured out they spoke in English. She apologized to them profusely, explaiining that they communicate through high and low note musical sounds.
Therefore, when Perch first sent that high note message, it means in their language "WE ARE TOYS". Also the low note musical message it kept telling them means in their language "I've come to play". The mother asked them not to think poorly of her son, because he meant no harm he just wanted play. Then together they vanished back to their planet.