Is it not the difference between fluid and liquid? The latter can be very viscous or "fluid"/free flowing?
Defn of fluid: a substance that has no fixed shape and yields easily to external pressure; a **gas or (esp.) a liquid**
It is a bit clearer if you look at the origins of the words.
Fluid (as an adjective): from French fluide or Latin fluidus, from fluere ‘to flow.’ Liquid: from Latin liquidus, from liquere ‘be liquid.’