To pray, we need to remove impurities from the body, cloth and place of prayer.
The Quran says:
‘Purify your clothing, and shun all filth!’ [74:4-5]
Washing with water is the main methods by which impurities are removed.
The Quran says:
“And We sent down from the heavens water that purifies.” [25:48]
Any other liquid substance can also be used to remove physical impurities, as long as it:
flows easily;
is itself ritually pure;
facilitates the removal of impurities.
Therefore, any filth can be washed off the cloth and place of prayer to make it suitable for worship. Apart from washing, there are other methods of purification, but they should be studied in more detail, so we won't mention them here.
A small amount of impurity is excusable if the whole combined area is less than 2 cm in diameter (like a medium-sized coin). If the area is more than that, then the impuritites have to be removed.
Practically speaking, we need to keep ourselves clean and pray in a clean place. Having a relatively thick prayer rug fixes the latter part very well.
Allah says in the Quran
“It is not Allah’s Will to burden you, but to purify you and complete His favor upon you, so perhaps you will be grateful.” [5:6]
Allah wants ease and purity for us, therefore don't go to extremes imagining that everything is impure. Don't investigate too much, ignore suspicions and doubts, and seek proper knowledge to know how to do things.