Although it intuitively makes sense to clean ductwork — after all, you dust and clean the rest of your house — the fact is dust that settles in your ventilation system generally stays where it is, unlikely to become airborne unless disturbed. Under most circumstances, the dust is inert and harmless, and stirring it up with cleaning equipment actually creates bigger issues.
You should be aware that although a substance may look like mold, a positive determination of whether it is mold or not can be made only by an expert and may require laboratory analysis for final confirmation. For about $50, some microbiology laboratories can tell you whether a sample sent to them on a clear strip of sticky household tape is mold or simply a substance that resembles it.
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