Use liquid detergent in water to wash the outside of the duct
(insulation). If it is less than 10 square feet that is all you need
to do. You can add a little clorox if no one in your house is
sensitive to the smell.
The real problem is that mold is a moisture problem. Why is there
moisture on the outside of the duct? Is there air leakage around where
the duct goes through the ceiling? Do you have a roof leak dripping
down on the duct in the attic? Is your blower speed set too low? Do
you have a dirty air filter restricting air flow. Do you have a
restrictive h(high efficiency) air filter? Are you running exhaust
fans constantly, which pulls humid outside air into the house to
replace the air you exhaust out? Are you running your indoor blower on
continuous - that will increase your indoor relative humidity 10 to
15%. Do you have an oversized AC? Do you have a plumbing leak? Do
you have a damp crawlspace or basement?
Finding out where the moisture is coming from requires answers to these
questions and more and requires a compentent tech. You have to look
for one of those, most techs have no clue when it comes to humidity!
If you clean up the mold and don't fix the moisture problem, the mold
will just come back!