Is there a difference between marshmallow fluff and creme? Can I use fluff if I don't have creme? Thanks!

You've got your answer but be aware that fluff/creme contains eggs where marshmallow does not and that is important to people with allergies (I bake for one.)

Yes, Fluff is a particular brand of marshmallow creme, so they're pretty much interchangeable in baking (even though aficionados claim Fluff is a must for fluffernutter sandwiches!).