Hii!
The main difference between a parasite and a saprotroph is that,
A parasite obtains it's food or nutrients from a host, whom it infects or lives on.
A saprotroph obtains it's food or nutrients from dead and decaying organic matter.
Thus,a parasite may or may not be harmful to.the host, but a saprotroph is always beneficial in keeping the environment clean.
Hope this helps.
Good luck!