either some type of fungus or alage, im leaning towards the fungus with the apperance and more so the color. i would take out all the decor and wash them off really well in some chlorinated water, i use bleach. then rinse them off really well aftewards and throw them in a bucket with some dechlorinator. this will kill off any fungus on the decor. are you seeing it only on the decor or is it all throughout the tank including the rock?