The Questioner: Is it permissible for us to make wuḍūʾ (ablution) with the water of a pool where the colour of the water has turned green?
Shaykh Muqbil: If there is a change due to people bathing in it and/or performing al-istinjāʾ (الاستنجاء – i.e., a means of purifying oneself after anything exits from any of the two exits), then it is not permissible, because it would have become impure (نجس – najis). As water is purifying (طهور – ṭahūr, i.e., it removes impurity), and nothing makes it impure except that which changes its taste (طعمه), colour (لونه), or smell (ريحه), as a result of an impurity interacting with it.
And if the change is due to how old it is [without use], then it is only something that is filthy [and unclean]. It is even true that a person might experience nausea if he rinses his mouth (مضمض – maḍmaḍah) and the nose (استنشق – istinshāq) with these pools. So I advise the owners of these pools to take pitchers for themsleves and to draw water from the pools and to go outside [to make wuḍū].
And Allah, Glory be to Him, the Exalted, will make it easy if He knows their and honesty and sincerity.
(Ijābat as-Sāʾil, p. 31)