Cladophora grows in short, green filaments that are quite tough. It typically attaches very firmly to hardscape in the aquarium, such as wood or stone. It can also grow throughout plants such as moss and carpeting species, creating a tangled mess.
Some hobbyists will purposely keep Cladophora, as it can look appealing when added to hardscape and carefully managed. In fact, marimo moss balls are a type of cladophora and are widely kept in freshwater aquariums.
If not intentionally introduced, cladophora can be caused by slow water circulation, areas receiving no flow and intense light, areas overcrowded by plants or decaying undergrowth. Generally speaking, low flow encourages this algae to form.
To treat, first manually remove as much as possible, which can be difficult. For mild cases, you can spot-treat with hydrogen peroxide misting or products such as LCA Carbon Plus or Triple B.
Any old or decaying growth should be pruned or removed. Similarly, overgrown plants should be trimmed and tops replanted to help with flow. If lighting is intense, consider adding more plants to use the light available.
Increasing growth rates of your existing plants is recommended by optimizing your pressurised CO2 (if using), and adjusting fertilisation rates if required.