White plague is the name of the compound Au5Al2 as well as the problem it causes. It has low electrical conductivity, so its formation at the joint leads to an increase of electrical resistance which can lead to total failure.
Purple plague (sometimes known as purple death or Roberts-Austen's purple gold) is a brittle, bright-purple compound, AuAl2, or about 78.5% Au and 21.5% Al by mass. AuAl2 is the most stable thermally of the Au–Al intermetallic compounds, with a melting point of 1060 °C (see phase diagram), similar to that of pure gold.
https://en.wikipedia.org/wiki/Gold%E2%80%93aluminium_intermetallic
https://en.wikipedia.org/wiki/Colored_gold#Purple_gold