Some of these algae make toxins that interfere with nerve signals in fish and people. Sometimes shellfish seem to accumulate these toxins without serious harm, but humans get sick after eating them. Other people get sick after fishing in infested waters. Algae is a general category of one-celled plants that are sometimes called phytoplankton. Here's a good description of the complex lifestyle of a harmful alga.
Some harmful algae can exist in sediment in a dormant form called a cyst.
Under the right conditions, these cysts can germinate and turn into the swimming stage that divides to form masses of algae that can be dense enough to tint seawater. Sometimes water that is perfectly blue or clear can be more dangerous than a visible red tide because even a small number of cells of certain species represent a lot of toxin. Whatever the name, these harmful blooms can carry potent nerve toxins. When people eat shellfish contaminated with these toxins, they can develop paralysis, amnesia and other neurological problems. Shellfish that make their living by filtering tiny bits of food from the sea are often the first victims of a harmful algal bloom.