If the patch is smaller than about three feet by three feet, you can probably deal with it yourself. That is the EPA’s own guidance: if the moldy area is less than about ten square feet, in most cases you can handle the job yourself. New York agrees from the other direction — the Labor Law only defines a "project" as work on areas *greater* than ten square feet, so at or below that, no licence is required and no licensed contractor is needed. The state Health Department gives the same figure. Scrub it off hard surfaces with detergent and water and dry it completely.
Two things change that answer. If you or someone in the house is sensitive to mold, both the Department of Labor and the state Health Department suggest getting a professional in regardless of size. And porous material that has stayed wet — soaked drywall, carpet, insulation, ceiling tile — generally has to come out rather than be cleaned, whatever its area.
You probably do not need testing either. The EPA’s position is that in most cases, if visible mold growth is present, sampling is unnecessary. New York’s Health Department goes further and says plainly that mold testing is not recommended, nor is it required by any regulatory agency, because there is no state or federal standard for mold in homes. CDC’s NIOSH does not recommend routine air sampling and says visual inspection and musty odour are more reliable. If someone is selling you a lab panel to prove you have a problem you can already see, ask what decision the result is going to change.
Be careful that this does not read as "skip the assessor". They are different things. On a job over ten square feet New York requires an independent licensed assessment before remediation can lawfully begin — that is an inspection and a written plan, and it is not optional. What is not required is lab testing. An assessor can write you a plan without sending anything to a laboratory.
And whatever the size, the mold is the symptom. The EPA’s whole guide comes down to one line: the key to mold control is moisture control. Clean it up without fixing the water problem and it comes back. New York builds that into the law — under § 947 the assessor’s post-remediation check has to determine, as far as is feasible, that the underlying cause has been dealt with so it is reasonably certain the mold will not return. A remediation that leaves the leak running can fail its own clearance. So even where you clean the patch yourself, the question worth paying somebody for is where the water is coming from.