I've been seeing something on my LR lately that I never saw before..... it looks like mold.... darker black/grey towards the center and almost white around the edges.... I've noticed it's moving... it's got no real shape and is smaller than a dime... but it doesn't seem to want to leave this one rock.... any ideas????? It's doesn't move fast enough to watch it... I just notice it moving from place to place...... anyone??????