"Alt-left" doesn't exist. It's a projection.

"Alt-right" is an actual term made up by an actual white nationalist named Richard Spencer. He did that to try to rebrand and mainstream his racist organization. He's trying to hide and obfuscate because their real moniker of "white supremacist" is rightfully tainted. Judging from Steve Bannon in the White House who bragged about his website being a "mouthpiece for the alt-right" and judging by the way Trump is elevating the alt-right by echoing their talking points and even retweeting out and proud white nationalists, Richard Spencer has so far succeeded. 

Meanwhile, communism isn't claiming to be "alt left". It's far left. Always has been. So marching under a banner of Workers Party is keeping with tradition of communist branding throughout history, not some obfuscating alt-branding. Same with socialism. Eugene Debbs was a socialist. Socialists today aren't trying to cynically hide their true intentions. They are proudly socialist.