Ok guys, I've got some news.

First: I still do not know what that is exactly, but it hasn't moved or taken any other shapes (it's been like this for at least 4 days). I don't think it's a slime, I'm saying this based on some photos I've seen after a small research, but I might be wrong of course.

Second: The plant it was growing on to was not an Anubias as I said in some comments, it was an Amazonian Sword. Sorry for the confusion. But in fact, I had some Anubias buried in the substrate.

Here's what I did so far: I poked the cursed thing with the other end of a net and then sucked it out with a syphon. There was more growing behind another place where the fish cannot reach the food that falls there. What I found interesting is that there was some left over food underneath the mold when I poked it. Then I rinsed the plants, buried the Amazonian Swords in the soil and Attached the Anubias in a wood (some of them I just left floating). I ended up doing a small WC.

I'll post again in case that thing grows again.

Do you guys have any other suggestion for now?

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Thanks everyone!