Describe difference between a stress fracture on the metatarsal, and metatarsalgia?

Not the same: A stress fracture is a break in the bone.Metatarsalgia is a catch all diagnosis for pain in the metatarsals.

Stress fracture: Is a break in the bone. Metatarsalgia is a general term for pain to the metatarsal head regions. Someone can have pain to the met heads from a skinny fat pad, or a bursitis etc it is a generic term.

Fracture: See my other answer on the history of the stress fracture. Stress or pathological fracture is a non displaced fracture . That is no bone shift takes place. Metatarsalgia is a catch all term for forefoot pain for a variety of causes or reasons.