It took weeks of searching the internet for answers with no luck. I just love the style of it so much. At first I saw someone say the signature looked like it alluded to something like "Chris Lang" but I couldn't find an artist under that name with a similar style.

I finally found my answer tonight when I went through the list of crew members who've worked on the show (yes, I spent my free time doing that, no I don't have a life.) And I figured it out!!!

Crosland. The signature belongs to artist David Crosland, who has worked on various episodes of the show and he has done fanart for Rick and Morty as well as cover art for Invader Zim comics.

His online portfolio is davecrosland.com if you wanna check out his stuff, I highly recommend it :)

TLDR: I spent way too much time looking for Dave Crosland, the artist behind the Wall murals.