Hiiro no Kakera was once a game for the PlayStation 2. It popped over to the PSP and even the Nintendo DS. It’s a sign of the depth of the plot that it’s travelled so well and is now coming to life as an anime. There’s usually a busy collection of Hirro No Kakera trades on eBay.
The synopsis does include a high school girl being saved from “slimes” by a mysterious boy… but they’re not likely to take the anime series to any level of naughtiness as the game was for 12 and up.