Cyanobacteria, or blue-green algae (BGA) appears as dark green slime. This algae starts as translucent sheets, often along the substrate, and hugging poorly growing plants. It can form filaments that trail in the current. It is often caused by a combination of deteriorating plants, excess organic waste at substrate level, poor oxygen levels, and low nitrates. BGA is actually a photosynthetic bacteria capable of fixing gaseous nitrogen. In large amounts, it smothers plants and plant growth can stall - giving rise to more issues.
Algae eaters often avoid this algae as it can be toxic.
A patch of cynobacteria / blue-green algae (BGA) on substrate, above.
Blue-Green algae appears as a slime across carpet plants above.
Here are links to further reading on:
1. Detailed steps on how to cycle a tank
2. Optimizing gaseous exchange and flow patterns