For some reason this didn't work for me :(
% xattr -d com.apple.quarantine PixelComposer.app xattr: [Errno 1] Operation not permitted: 'PixelComposer.app'
Tried running the command with sudo, but no luck either.
But I found another workaround -- I was able to execute the `Contents/MacOS/PixelComposer` program inside the .app folder directly. I've posted the full command in a top level comment below.