Dark smoke is probably just the SCP attempting to manifest as something and failing. Given that multiple people can perceive it as something different, it's likely a hallucinogenic being emitted.
I know there's probably another (possibly better) manga out there that does the whole "can't understand each other's language" thing, but to me it's a novel concept. Plus, I like the plot points being laid out so far, even if some people can see them. Easiest $2.50 spent for 150ish pages.
Not many obvious errors, just weird english stuff (when listing groups of people it is "person A, person B, and I", never "me, person A, and person B" or "person A, person B, and me") and some formatting issues. Pretty good overall!