A molly's bite does not hurt, does it? The only reason I would be alarmed would be if your hands were dirty or had some chemicals on them that the fish might ingest (hand soap, laundry detergent, etc). Mollies are not the most intelligent species of fish and the only reason I can come up with is that it is being playful, maybe because it's lonely. If a Jaguar Cichlid bites your hand, (which by the way, does hurt) it is trying to say "I'm distressed" or "You're invading my space" but a molly is not territorial nor is it lble to communicate stress through communication other than fading colors or something like that. But never personal interaction.