Mold doesn't grow well outside of around 60-80F (15-26C), and it needs food. Steam and hot water systems also use treated water to help reduce corrosion in pipes. That said, radiators themselves still do corrode and create nasty looking water even if it's generally sterile.
You should be more worried about Legionella, but proper steam and hot water systems will stay above 140F (60C) as that temperature not only prevents the bacteria from multiplying but will kill 90% of them within 2 minutes.
You should be more worried about Legionella, but proper steam and hot water systems will stay above 140F (60C) as that temperature not only prevents the bacteria from multiplying but will kill 90% of them within 2 minutes.