What scum is that? is an App to help identify potentially toxic freshwater algal blooms that may be forming scums or colouring the water. The App leads you through a series of questions to identify your algal bloom and determine whether it may be a potentially toxic blue-green algae. The app also has some background information on algal blooms and allows you to report your bloom. It is part of the Algal Resource kit developed by UTS with funding from the Environmental Trust.