What is the white mold/fungus in my freshwater aquarium?

I came home and added some water to my aquarium (30 gal, about 10 semi-aggressive fish)

I noticed there’s a whitish mold growing on the pebbles on the tank floor. It almost looks like cobwebs.

Also, all of my fish are fine — except I noticed my two plecostomi are dead. (They sit still for so long, I don’t know how long ago they died.) Could they have eaten this mold, and it killed them?? What is it, and what can I do to get rid of it??

Thank you!!

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