If your crema is less than 48 hours old, just mix it up really well before serving. This will get rid of its unappealing brown tinge (which is likely isolated to the top layer). The best way to deal with avocado and its tendency to turn brown is to prevent it before it happens. Use plastic wrap to cover your crema as it will create a barrier that helps keep the air out and your crema looking fresh for as long as possible. Just apply plastic wrap directly to the surface of the crema.