What is the moral element in Mansfield Park by Jane Austen that makes it a moral novel?
If I understand your question correctly (which I rewrote a bit), you want to identify the elements of Mansfield Park that lead some to classify it as a novel about morality, or a moral novel. There are many elements of the story that deal with moral questions, in fact, one might make the case that moral questions form the bedrock of the thematic concerns of the novel. Some of these moral questions are still recognized today as moral issues while others have faded a bit from the moral limelight (though not altogether).
The largest questions of morality center around the behaviors of the two sets of siblings: Maria and Julia Bertram and Henry and Mary Crawford. Actually, these have mirror opposites in the persons of siblings Fanny and William Price. The Bertrams and the Crawfords illustrate immoral behavior as it relates to misconduct between persons of opposite sex, including the elopement and estrangement of married persons. When Edmund tells Fanny how he and Mary talked this over, he is outraged that Mary calls it "folly":
To hear the woman whom—no harsher name than folly given! .... She saw it only as folly, and that folly stamped only by exposure. .... it was the detection, in short—oh, Fanny! it was the detection, not the offence, which she reprobated.
Other moral questions on the romance side are illustrated by the scheming plan Mary has to capture Edmund and the difficulty Fanny has in enforcing her refusal of Henry's marriage offer:
You think only of yourself ... without wishing even for a little time to consider of it, ... are, in a wild fit of folly, throwing away from you such an opportunity of being settled in life, eligibly, honourably, nobly settled, as will, probably, never occur to you again.
Other weighty questions of morality are projected into the discourse. For instance, Sir Bertram is asked by Fanny about the "slave-trade." Some critics say the "dead silence" is more expressive of Bertram's beliefs than a recorded response would have been: his "dead silence" indicates a complete negation of the moral or human importance of the "slave-trade."