RedAcan

What you have sounds like "Cyanobacteria" which can be green or red. You can treat with red slime remover or antibiotics since it's a bacteria not an algae. Stirring will only spread it, but it is a good idea to suck up and remove as much as possible before treating. Here is a link to learn more http://kb.marinedepot.com/article.aspx?id=11162