Hello. I would like to clarify some details. It is well known that Rambam is often pasken like Yerushalmi and not like Bavli. But in his “Isurei Biah” 10:9 about what is “tzuras adam” Rambam quote rabbi Yochonon from Bavli (Niddah,23b) but in Yerushalmi there is opposite opinion in name of rabbi Yochonon. So why (and how) did Rambam pasken not like Yerushalmi in this case?

In general the Rambam paskens like the Babvli except for special cases such as a case that the Bavli doesn't pasken and remains in a safek and the Yerushalmi decides the halacha without a doubt (see Shach Yoreh deah 145; 1). My friend and former chavruta Harav Achikam Kashat brought different sources about this. See his article here All the best!