Morphea (morphoea) is an immune‐mediated disease in which excess synthesis and deposition of collagen in the skin and underlying connective tissues results in hardened cutaneous areas. Morphea has different clinical features according to the subtype and stage of evolution of the disease. There is currently no consensus on optimal interventions for morphea.
[…[
Morphea is a rare disease that causes skin hardening. It affects adults and children equally, and is more common in females. There are different subtypes of morphea, with different characteristics: circumscribed morphea is generally less severe than the other subtypes; linear scleroderma can cause significant body differences, possibly affecting growth in children; generalised morphea is a severe type involving multiple areas of the body; pansclerotic morphea is a severe and progressive type of generalised morphea; and mixed morphea is the presence of two or more disease types.

Source : Cochrane Database Syst Rev. 2019 Jul; 2019(7):

It's thought to affect around 20 to 40 per million people, but may be under-reported.

Although it's known to be immune-system mediated, the cause of morphea is currently unknown.