I am a very soft mineral and rich in magnesium and silica. I have a soapy feel and I am often green. I am used in baby powder and lipstick. What am I?
I would go with talc, which not only has the uses mentioned in the question but also is defined to have hardness 1 on the Mohs scale.
Talc is soft because it's magnesium silicate based structure consists of layers that are only loosely bound to each other and thus slide easily. This property makes talc a good lubricating agent, as well as soft, and so drives its practical uses.
For more on talc, see https://en.wikipedia.org/wiki/Talc.