What is the difference between kingdoms protistae and protoctistae?
First, just a correction: it's "Protista" and "Protoctista", not "Protistae" and "Protoctistae".

They mean the same thing. Those are just different names proposed by different taxonomists for describing the same group of organisms.


Protoctista (or Primigenum) was proposed by John Hogg in 1860.
Protista was proposed by Ernst Haeckel in 1866.


However, as it is common in taxonomy and nomenclature in general, things sometimes get a bit more complicated: some biologists, like Lynn Margulis, don't treat those two terms as synonyms. Particularly, I don't use her nomenclature.



Source: 


Jstor.org. (2017). Protozoa, Protista, Protoctista: What's in a Name? on JSTOR. [online] Available at: http://www.jstor.org/stable/4331095 [Accessed 12 Sep. 2017].