In 5e, if a Grease spell is cast and a large (eg. 10'sq facing) creature is killed on it, covering the same area as the spell, could other players/NPCs/monsters walk over that creature as if the Grease spell does not exist?
The rules do not say
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 the books, 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.
Large creatures are not 10ft wide
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)
Keeping in mind that a large creature isn't a 10 x 10 square object, the DM may, for example, call for a Dexterity (Acrobatics) check for walking along the irregular surface that is the prone body.
Nothing states that you can't though
"There is no rule for that" does not equal "you can't do that". Player characters can (and probably should) do perform actions not detailed elsewhere in the rules:
When you take your action on your turn, you can take one of the actions presented here, an action you gained from your class or a special feature, or an action that you improvise.
When you describe an action not detailed elsewhere in the rules, the DM tells you whether that action is possible and what kind of roll you need to make, if any
(PHB p. 192 , emphasis mine)
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 provide a sound surface, it probably negates the spell effect (but still, ask your DM).