Some are, some aren't. Upon further research it does appear the type used in this algal flour are related to plants but many algae species aren't closely relate

Some are, some aren't. Upon further research it does appear the type used in this algal flour are related to plants but many algae species aren't closely related to plants at all. https://en.wikipedia.org/wiki/Viridiplantae https://en.wikipedia.org/wiki/Viridiplantae