Strange white Spider Web things

[spuds]

Im curious as to what these are. My tank has been up and running for 4 months and Im noticing these white spider web like things growing on my back glass, power head, and thermometer. They range from 1"-2" across and apear to be spreading. What could these be, is it bad?

[tony]

i forget what they are off the top of my head but they are not bad