Eliminating dirt, dust and microbial growth from your A/C duct will not only improve indoor air quality, but it will also increase HVAC efficiency.
Life in the tropics often means that A/C ducts and vents will be susceptible to mould and bacteria growth if they are not regularly inspected and thoroughly cleaned by a trained professional. It is hazardous to health and must be removed quickly if discovered.
You should get your maintenance technician to regularly photograph inside your ductwork to keep an eye on mould growth. Unfortunately, fiberglass ductwork cannot be safely cleaned once it has mould growing in it, so it must be replaced. If your roof space is insulated with spray-foam insulation, you can get KoolDuct, which does not allow mould to grow.
The following companies offer mould remediation, decontamination and A/C duct cleaning and sanitation: