Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
Mustard seed in a dream is unlawful money or poison. Eating it indicates falling into worry or illness.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Mustard seed indicates a man of bad character. Seeing it indicates worry and grief.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
Mustard seed is interpreted as worry. Whoever sees himself eat it will be afflicted with worry.