Indicators of mold in an attic do not necessarily mean mold growth or activity. Excessive water vapor must be present for that to happen. Mold spores that grow on wood start out in a circular manner then send out feeding tubes that resemble fingers on a hand.
Insulation traps water vapor. Particularly with dirt basements, water vapor will migrate through the house eventually settling in the attic. Each layer of added insulation will trap more water vapor which will create an environment for mold growth. A dirt basement would have to have a vapor barrier installed and an adequate attic ventilation system would have to be improved upon or installed to handle the effects of additional insulation.
In nearly 100% of the attics we remediate, additional insulation was the culprit. If you call an insulation company to add additional insulation, they will likely request that you have your attic remediated for mold (even if it’s not necessary) before they install the insulation, leaving any responsibility for new mold growth to the homeowner. Before you consider having insulation installed, take care of the vapor and ventilation issues first.