Symbiotic relationships mean that two individual organisms live together while each derives nourishment from the other. It is amazing at how some organisms large and small, colorful or dull, life giving or life getting, manage to go through their life spans living off the productivity of others. In the algae family, this is called symbiotic living. That adjective, "symbiotic" separates this kind of mutual living arrangements from the other less desirable phagocytes. Their kind of life arrangement means one organism - again, large or small - just mooches off the other and gives nothing back. In other words, they pay no rent.
Algae are different. Often they are thought of as a branch of fungi but there is one big difference; Algae are oxygen producing and sustains life of other vegetation and small aquatic animals. While they are the low men' on the food chain ladder, they are the primary life givers. In this capacity they are mammoth. No discussion of their symbiotic relationship with other organisms is complete without mentioning they are being primed to help save this planet from chocking on its own overabundance of carbon dioxide.
How can they do this? All they need is sunshine and water to create oxygen. This arrangement is easily managed and voila! You now have a simple green machine that can utilize that overabundance of carbon dioxide by turning it into oxygen. That's basically what happens here in this symbiotic relationship between algae and its environment.
Kelps and seaweed are algae as are that green fungul looking stuff that grows on rocks and on trees all around. It is adaptive living in salt water or fresh water ponds, in sand, in icy places of the world and, in fact, in some form everywhere. They are in essence, rescue squads for the castoffs of humanity. Algae, of course, can be characterized by their use such as good or bad algae, but here we concentrate our effort on the green algae, which in itself is like wise plentiful. It shares a feature with plants in that it has two kinds of chlorophyll; yet unlike plants green algae grows in water.
Without getting into the jargon that associates itself with academic studies and without delving deeply into the characteristics of algae, suffice to say algae is a primary plant and is at the beginning of the food chain. It supplies oxygen to whatever surrounding material it grows on.
Green algae such a sea lettuce that grows in water has much in common with the earth's moss. They both are the first green oxygen producing organisms at the beginning of the food chain.
Is it any wonder then that on the north side of trees where there is little sunshine there grows moss. That says to me that this is symbiosis. Although we are speaking of moss and not of the topic algae, or put in another way a land growing algae as opposed to a true algae growing in water, both have similarities. Both need dampness and water to grow.
Note: In all sincerity, due probably to my lack of intense research, I did not find any direct reference to symbiosis where algae is concerned; therefore most of these comments are speculative in nature. I will be glad to learn of my errors should they be found to be incorrect. There is so much all of us have to learn about the abundance of nature and its resources - or lack thereof - that we need all the help we can get when trying to decide the difference between algae and moss. This is especially true when one is only a casual observer.