Hello, My tank has been running since December 27th. For the past two weeks or so I have noticed this purple hairy looking stuff growing on the rocks and spreading to the sand. During my water change I was able to siphon it off the rocks, but it grows back. In the morning after the lights have been off overnight, it seems that it has died off, but once the lights go on it appears again in a few hours. What is this growing? All of the fish seem fine, but can this harm them? Thank you for your help.