What's the difference between social policy and public policy?

Social policy is one subset of public policy.  So all social policy is public policy, but not all public policy is social policy.

The term "social policy" is typically used to refer to policies that have to do with people's welfare.  These would include policies affecting public health, education, criminal justice, and other such things.

Public policy, on the other hand, can refer to any policy taken by the government.  Public policy that would not typically be seen as social policy would include such things as environmental policy or tax policy or the space program.  Of course these things affect people's welfare, but they are not so directly connected to it.