Rav belskys heter was on multiple fronts: he held that it might not be considered visible, since it’s only visible against a black background (rav dovid feinstein held that this is considered visible), he also held that it’s not kavuah and you can rely on rov cups not containing cope pods. A 3rd heter is that it’s not a forbidden sheretz to begin with, because it was never shoretz al haarerz (interesting to note that based on this, he held that filters can potentially do more harm than good, because if there’s a hole and one gets stuck, it is now shoretz al haaretz and would be assur).
His heter was robust, but it was not on his list of shitos that made him not hold of a person who argues (the eruv and worms in the fish are on that list)