You found mold — the first steps

In the first day

1. Don't disturb it — scrubbing or knocking into it can spread spores.
2. Find and stop the water if you can (a running leak, a dripping trap, a downspout dumping against the foundation).
3. Turn on ventilation and reduce indoor humidity where practical.
4. Photograph what you see and where, in case of an insurance question.
5. Call to be connected with a provider who can identify the source before anything is removed.