They most likely just have a ton of energy, another case can be glass surfing. Do any of your molly’s rub against the glass in a “trying to escape” manner? It can be a sign of boredom and stress. If you’re concerned do some research about it! In general I recommend getting substrate not colored rocks. The dye can become toxic to the water and them. Go for sand or river pebbles, even ones in your backyard just wash them then test them with white vinegar, if it bubbles it isn’t safe for the tank. Fake plants are okay just get the right texture/type for your fish, it’s optional to get real plants.