Unless you have paper backed, or wool insulation, mould does not decompose fiberglass insulation.
Most often mould is found on the dust in the insulation, sometimes like the picture above the dark areas are dust caused by thermal bridging (air infiltration) and not mold.
It’s always safe to discard the insulation to prevent the dust to develop fungal growth if moisture develops.
Tip: If the darkness looks fuzzy, slimy or smears when pressed, it is mold and you SHOULD discard it. Large amounts of mold in the insulation require professional remediation and should not be removed without proper precautions to ensure safety
If that sounds like you, call us for a free inspection at 416-791-8020.