Alright seems a simple Google search would have done. It was Tobias, as clarified in this article:
In Infinity War, Star-Lord, Gamora, Mantis and Drax return to Knowhere
in hopes of prying the Reality Stone from The Collector before Thanos
can get his hands on it. However, they arrive to find widespread
destruction. When they hear Thanos’ booming voice, they try to creep
up on the Mad Titan, and Star-Lord signals for his teammates to stop,
but no one does. As Gamora walks away, the top left corner holds the
mother of all Easter eggs: Tobias Fünke in his Blue Man Group makeup —
white cut-off shorts and all. He’s one of The Collector’s specimens in
the terrarium-like containment cells. Now, just to be clear, it’s not
actor David Cross, but it’s unmistakably Tobias.