Blue-Green algae are a bit different from other algae like the diatoms and the green algae. Their cell structure is more like bacteria than like plants, and this is why they are classed with the word bacteria in their proper name Cyanobacteria. They grow in all types of water from fresh to sea water, with all the slightly salty waters (brackish) as well.
There are a couple of differences between a diatom and a blue-green algae bloom. The first is that there are many more species of toxic cyanobacteria that cause problems. With diatoms, only the species, Pseudo-nitzschia australis, is likely to cause a toxic bloom. The most important differences is that with a diatom bloom of any kind, when the cells die they settle to the bottom of the pond or dam so the surface remains clear for oxygen exchange, whilst at the bottom of the pond the water may become anaerobic (no oxygen) as the dead cells decompose.