The fish in the photos is a female, and she's pregnant.
Can I suggest you take a close look at the other mollies, specifically at the anal fin. That's the one right in front of the tail on the under side. Females, like the fish in the photos, have a fan shaped anal fin. Males have a rod shaped anal fin. If it was the shop worker that selected your fish, they either caught a female by accident or they can't tell the difference so you need to check exactly what gender each of your fish is.
If the other 5 mollies are males, you do have a bit of a problem because these fish, like the other common livebearers, should be kept with at least 2 females for every male. The reason is that male livebearers constantly pester females, and you need more females than males to give each female time off while other females are chased. With 5 males to 1 female, she will be extremely harassed.
