They have 3 other dogs besides Mojo. If we're going to assume Rocky is dead based on those pawprints, we should also assume Cleo and Mickey died with him in some kind of horrible doggie genocide. Don't worry.
I do think they would tweet RIP if he died though, they know fans love him. As of last summer Rocky was definitely still alive because Billie mentioned him on the Jeff Matika Show.
Billie's hair follicle could be on a piece of tape and it would be Green Day related