They are a good sign. Its a form of tube worm, an indicator of a healthy/maturing saltwater tank. They usually grow in very little to absolutely no lighting. I dont think you will find them in areas of high light, or on glass that doesnt have a background on the other side.
Actually forget what i just said, i just noticed that what i was talking about is not what you were talking about. I was talking about the small white circles on the center of the LR. The stuff growing on the outer right edge of the LR could be a sponge or some sort of macroalgae. Im not really sure.