Betta Fish: What on earth did I find in my aquarium?!

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This is the second one I've seen in just a few days. It’s somewhat firm and attaches firmly to the glass or decor using two transparent "suction cups" at the ends of its body. It doesn't move at all, doesn't react to being scraped off the glass, and shows no reaction when taken out of the water. Honestly, it looks a bit like a cocoon.

I can't find anything like it in the Google. Has anyone come across this before? 😖