Weird substance found in fish tank!?

I was cleaning my fish tank when I saw this GLURP stuck on one of my fake plants. I took it out and cleaned and put it back in, but I am still worried about the glurp. It was sticky and stretchy like mucus, but a deep brown. None of my fish have died recently, and I do the water change every 2 weeks, as I was told. What is this weird GLURP and how do I prevent it from coming again? Also, I found clumps of it stuck to a bunch of pebbles. WEIRD