Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
Ash in a dream is unlawful money or false, useless speech. Collecting it indicates striving for a false matter. Falling into it indicates worry from a ruler.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Ash indicates knowledge that is not acted upon. It may also indicate dispute and evil.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
Ash is interpreted as worry and grief. Whoever sees himself cleaning up ash is getting rid of a worry.