What is the meaning of the statement, "It is possible to guard against it, so it does not alter one of its characteristics," concerning pure water that has been mixed with something and had one of its characteristics altered?

The meaning of "that which is difficult to guard water from" refers to things that cannot ordinarily be kept out of water, such as algae and tree leaves near wells. If something that is not difficult to guard water from (like soap and shampoo) falls into it and changes one of its characteristics, scholars have differed on the permissibility of purifying with it. As for that which is difficult to guard water from (like tree leaves and algae), if it changes the water's characteristics, there is no disagreement on the permissibility of using it for purification, except for what has been narrated from the Malikis regarding tree leaves.