If it's not mould, don't worry.

Eventually, you'll have lots of mold because that outdated way of insulating basements invites and encourages it. But it's still legal in most US jurisdictions.

Kudos to the inspector for knowing the problem. Go further, if they buy the home: replace the fibrous insulation with foam boards or spf. In fact, what they have can be reused to the interior of the wall over the proper insulation.

Ditto the rim joists, again insulated with moisture-sensitive fibreglass instead of a spray foam or foam board.

Building Science Corp.