Here’s the spell description:
Slick grease covers the ground in a 10-foot square centered on a point within range and turns it into difficult terrain for the duration.
When the grease appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw or fall prone.
There is no rule “grease spell effect is negated when a large creature is lying upon” in any official source book. We on the internet can only tell you that spells do based on their description only, because we are not DMing your game. We cannot change or expand upon the rules and say “this is part of how the spell works”. We do not have that authority; the DM of the game does, and we aren’t that.
The Grease spell covers 10-foot square, and a large creature does not cover 10-foot square:
A creature’s space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions. (PHB p. 191)
You can try to walk over it, but the result will depend on the situation — how exactly the creature is lying, what body does that creature have, et cetera. This is the DMs job to adjudicate such things, so it becomes exactly the “ask your DM” type of question.
One thing we can say though — there is no “fluff” ignorable text in that description. It says “slick grease covers the ground”, therefore this slick grease is the exact reason why a walking creature “must succeed on a Dexterity saving throw or fall prone”. If you completely cover the grease with something big and heavy enough to be a sound surface, it probably negates the spell effect (but still, ask your DM).