That is 100% a mistranslation, then. Here's the text from the translation:
"They're known to filter feed and remove pollution. However, upon overgrowth, they cause pollution. So, they have to be regularly culled."
This is found on Page 6. They don't
cause pollution, they regurgitate it. That's the key mistake here.
Here's the raw:
and here's the original text stripped from the raw, and my slightly edited MTL translation
[彼らは水中の汚れを吸収する性質を持つが・・・] "They have the property of absorbing impurities in the water..."
[溜め込みすぎると今度は汚れを放出してしまう] "If it accumulates too much, it will release the impurities."
[なので定期的に討伐を行わなければならない] "Therefore, it is necessary to carry out subjugation regularly."
[汚れ] "Kegare" means "dirt", "impurity", "pollution", so they aren't entirely off-base with referring to it as pollution, but what the slimes feed on is not "pollution" in the sense that is understood in English, where it is something introduced to the water system from outside of it, but any sort of impurities in the water whatsoever, including things like fish waste or similar.
[溜め込み] "Tamekomi" means "hoarding", with the concept of "to store and put away". [すぎる] "Sugiru" means "too much". [今度] "Kondo" means "next time" in the sense of the near future.
So it really is just talking about slimes accumulating impurities then ejecting them when it gets to be too much; almost like the next stage of evolution from pearl oysters. Nothing to do with "overgrowth", which would be directed towards too high of a slime population.
Pinging SS_TT so they can see this as well.