In small amounts algae is very beneficial as it’s a brilliant food source for just about everything in the ecosphere and is present in almost all natural water bodies. However, when it blooms it can ruin an ecosphere. With enough mass, it can outcompete almost any plant and will completely fill your jar. At night it will absorb oxygen, using the process of respiration, which will kill off a lot of organisms. This reduces biodiversity and upsets the food web. On top of this, when algae blooms die off, a large amount of bacteria is needed to decompose the dead algae, which further uses up oxygen and thus kills even more organisms. Another algae like organism, known as cyanobacteria which is characterized by a vivid blue-green film does this process to an even greater degree and should be removed immediately if identified. (It is important to not that cyanobacteria is not an algae but a type of bacteria. I have however decided to include it here because I have no idea where else to put it.)
The best way to avoid algae is to have a decent amount of plants in the ecosphere. They will absorb all the nutrients, leaving none for the algae to grow on. This is why it is best to get plants in the ecosphere as soon as possible. It is also important to not give your ecosphere too much light for too long. Another effective method to keep algae at bay, is the use of amphipods (see Amphipods (Amphipoda sp). Please note that this is not effective against cyanobacteria.