That “write what you know” adage is a mixed bag of cats, in my opinion. The technical word for it that they whip out in English classes is verisimilitude — the feeling of truth in fiction. That’s well and dandy, but some people seem to think that means you can’t write about anything that doesn’t happen in real life.
I remember reading a picture book with my second grade class full of vocabulary words. It was a retelling of the Cinderella story, but with dinosaurs. Dinosaurs! It was the best thing in the world. (It’s called Dinorella: A Prehistoric Fairy Tale. Talk about a blast from the past, eh? Wink wink, nudge nudge.) It was fantastic, in every sense of the word.
Speaking of, there are entire genres, thousands of stories, devoted to turning “verisimilitude” on its head. Do you think Tolkien ever met an actual, scaly, fire-breathing dragon? He wrote five or six of the darn things. I have never been on an intergalactic spaceship in my life, but darned if I’m going to let that stop me from writing a space opera. I’ve never met a fairy from the Summer Court, but I wrote a book about them. So what if these things aren’t real right now? That’s the thing about words. You can do anything with them. If you want to write a book about cowboy aliens feuding with mermaids, you can do that. If you want to write a book about a cactus’s search for love, you can do that. It’s your brain, dude. Go nuts.
But now that you have your crazy cool world, it has to be relatable. This is what I mean by a mixed bag — there’s got to be some element that the readers can recognize and identify with. I don’t care whether your protagonist has tentacles. Maybe blue spots lighting up on his face is the alien equivalent of a blush, and he can’t stop glowing when he sees the other alien down the hall. Or maybe the protag is a dragon who’s trying to outdo that green-scaled idiot across the mountainside in a contest on whose lair is the most bedecked with jewels.
And not just the characters, the setting, too. Even Mars has crunchy sand underfoot that gets everywhere and annoys the crap out of your characters. Playing in fantastic genres is a lot of fun, but it won’t work if the only cool thing is the genre itself.