The term political economy far predates Marx. I've not heard of Marx famously using it, but I don't doubt he did use it since it was an established field of inquiry at the time--the predecessor to the modern field of economics.
Political economy as an area of inquiry took off in the late 18th century--in no small part due to Adam Smith--and was already very well established before Marx was born [1]. Absent evidence to the contrary it seems questionable to assert that Marx's use of the term was anything other than the common use of the time.
[1] https://books.google.com/ngrams/graph?content=political+econ...