There are some alcantara cleaners available. Not all work as advertised, but if you need just clean basic stains most of them work well.
I've got s-line half-leather seats to my A3 with perforated alcantara two month ago. The right seat was very filthy. It looked like someone used a passenger seat as a table. I used Gliptone Gentle Cleaner and cleaned most of the spots, but one 4 cm spot right in the middle that looked like someone spilled mayonnaise or ice cream remained, just faded a little. The spot felt (still feels) firmer as if the thing soaked through the alcantara and dried. I bought then Sonax upholstery & alcantara cleaner. It's foam. Used it ones. It has helped, though not 100%. Sonax suggests to use it a few times if the first one wasn't fully successful. I have to try again. But if anyone knows a way to get rid off something unknown that sits fast inside the alcantara, I would love to hear it too.