On the surface, the official 'Rosario + Vampire' manga and most adult fanworks look like they share the same characters and outfits, but the intentions and limits are wildly different. The manga is a shōnen romantic-comedy with supernatural action and generous amounts of fanservice; it teases, flirts, and builds character relationships across arcs while keeping explicit sexual content out of the pages. You'll get blushes, wardrobe mishaps, and ecchi humor, but the plot still prioritizes emotion, battles, and Moka's dual nature rather than straight-up erotica.
Fan-made adult works, whether doujinshi or explicit art, exist to explore fantasies that the original material wouldn't. They often amp up the sensual presentation, push characters into sexually explicit situations, or rearrange ages/consent to suit a fetish (which is a real ethical concern in some circles). The art style can either be rough-and-ready or hyper-polished, and creators commonly focus on particular ships or kinks, not long-term story development. Personally, I treat the manga as the canon emotional ride I fell for and see adult fanworks as a separate, often controversial creative outlet that I dip into only with a clear boundary between fiction and reality.