Bike pedals turning when coasting (not a fixed gear)
Most likely it is drag in the free hub / freewheel.  You can start with breaking it down and cleaning it and lubricating.  There are videos on YouTube.

Saw the update to the question.  Does the freewheel spin freely.  Of the bike if you spin it will it make 3+ revolutions.

What is all the brown stuff?  Clean and lube in general.