We often use the indefinite article when we're being indefinite ourselves.
The sofa is russet-colored. Maybe we should paint the walls a dark green?
Some shade of dark green, the particular shade not yet decided upon.
So, if we mean to say there's some liquid on the floor whose makeup has not yet been determined, "a liquid" would be suitable.
If we tell someone "there's a liquid on the floor", it means we don't know what kind of liquid it is. If we know it's water, or chlorine bleach, we wouldn't say "a liquid".
That said, if someone asks us why chocolate M&Ms melt in our mouths and not in our hands, we can say "Because they have a candy coating". There, "a" simply means candy coating not of any particular type. We don't need to know that it is one that contains carnauba wax. We could say "a carnauba wax candy coating", and there we'd mean "some kind of candy coating that contains carnauba wax".