Does billie joe Armstrong know about fanfic or slashfics?

Yes. He's read a few. Here's a quote from him about it, "Tre reads the dirty ones. I've read a few. It's flattering and sort of weird at the same time. I don't really know what to think about young girls thinking about old men like that. But the ones I have read were very well-written and as long as kids these days are still being creative and writing, then I think it's cool." -Billie Joe Armstrong.