From Muppet Wiki

Trianglebob Trianglepants is the star of a TV show of the same name on Sesame Street. Triangle aficionado Telly is a fan of the show.

Trianglebob is a spoof of Nickelodeon's popular cartoon character SpongeBob SquarePants, and is portrayed by Bob McGrath in a colorful, triangle-centric costume.

The script is set when Telly comes into Hooper's Store telling Alan to turn on the TV, because it's time for Trianglebob Trianglepants on Triangleodeon (a spoof of Nickelodeon). Alan likes the show too and it comes on again in three minutes.