White webbing-like substance in tank

[BCTugboat]

Hey again guys and gals, I just moved a macroalgae and a piece of live branch rock in my tank and yhe water was suddenly full of this thick webbing material. I moved things so I could "plant" my new feather duster worm. I can't figure out how to upload a photo at the moment but if someone lets me know how I can do so. I have a 3.5 gallon Pico set up. I've heard it could be a type of "mat bacteria". But thats generally associated with die off on live rock. I didnt cure the rock. It came from a dedicated live rock tank at the LFS and was the first thing in the tank. Its been set up 3 or 4 weeks with just a brittle star I got by accident on my macroalgae. Any ideas of what this stringy material is, or if its harmful, are appreciated.