I upgraded to macOS 10.13.2 (had been meaning to get around to it eventually anyway, so figured I might as well see what happens) and I no longer get this error, so whatever the problem might be 10.12.X and earlier?

Same problem here. On OS X 10.10.5 if it helps.

Things I've tried: reinstalling game via steam, restarting steam, restarting laptop & steam.

Not sure how to adjust sprite resolution & VRAM usage; can't open game to get into graphics settings.

Process: factorio [532]
Path: /Users/USER/Library ...