Today I notices some dark red growth on inside my aquarium. Any idea what this is?
It's cyanobacteria. It comes in a multitude of colors, purple'ish and dark green are the most common, but can be any range from tan to almost completely black.
If this is a new tank, it's not uncommon to go through a number of algae and bacteria stages over the first year or so. In this case it will likely pass if you keep up good maintenance. You can blast it off the rocks and suck it out with a turkey baster if it is loosely attached.
Otherwise, it's a result of either too many nutrients (overstocking, over feeding, poor maintenance, or a combination) or an imbalance in nitrate (NO3-) and phosphate (PO43-) levels. It's very common a few months after people start a carbon dosing or biopellet reactor, as the NO3- is brought down to nearly zero while PO43- remains at a higher level.