Cleaning the masjid honours Allah's houses. The Prophet ﷺ ordered masjids be built in the neighbourhoods, kept clean and perfumed. (Abū Dāwūd — ṣaḥīḥ). A person who used to sweep the masjid died; the Prophet ﷺ prayed at the grave, valuing that service. (al-Bukhārī). He forbade spitting or leaving filth in it.
العربية: عن أبي هريرة رضي الله عنه أنّ امرأةً كانت تَقُمّ المسجد ففُقِدت، فسأل عنها النبيّ ﷺ، فقالوا: ماتت، قال: «أفلا آذنتموني؟ دُلّوني على قبرها»، فأتى قبرها فصلّى عليها. (البخاري).
Pick up litter and keep the masjid pleasant — never leave any harm in Allah's house.