Thanks for that. I usually put cellophane wrap right down on the top of homemade guac and that keeps it green for a day or so but this was guac from a restaurant doggie bag and in a cardboard container. I didn’t think to cover it but enough of the bottom was still green and flavored my leftover quesadilla nicely.
It’s not harmful, but it’s ugly in appearance and negatively affects the flavor. Guacamole is such a fleeting food. Lemon juice can help preserve it as can placing plastic wrap directly on the surface of the guacamole in a bowl. Otherwise you can just scrape the brown part off. If it’s not for a party, I make guacamole in small amounts rather than try and preserve a large bowl of it.