Elmo or Mo-Mo

On Sunday, Daddy and Caleb were playing with Caleb's stuff toy, Elmo, and Caleb just started calling Elmo by his name. He said Elmo twice.
Yesterday, Mummy tried to see whether Caleb really knows Elmo's name and pointed to Elmo and asked Caleb "who is this?" And Caleb said "Mo-Mo". He was calling Elmo "Mo-Mo" the whole day.
Today, Mummy asked Caleb again and the reply that Mummy got was both "Elmo" and "Mo-Mo". I think little Caleb thinks that "Elmo" is the formal name and "Mo-Mo" is the pet name that he is giving Elmo.

Here's a video of Caleb saying Elmo and then Mo-Mo.