Islamic Interpretation of Dreams (Ibn-i Sirin):
(Mustard; Money; Poison) In a dream, mustard means poison.
If one eats mustard in his dream, it means that someone may poison him, or that he may fall into a vicious circle, or acquire an evil reputation.
It also means that he may receive clean money, though through awesome hardships. Mustard seeds in a dream only denote good if seen by a physician. Otherwise, for the majority of people, seeing mustard or sesame seeds in a dream means a sickness, a fever, or poison.