Use the \times LaTeX operator instead of '*' or \ast. There is no means available to align the asterisk with the rest of the equation.
The Pages equation editor is simply the BlahTeX™ LaTeX to MathML converter, and is very restricted in those aspects of TeX/LaTeX that it supports. You can read more about that here, and here. Substitute de-de for en-us in the link if you prefer native German content.The second link will give you access to the blahTeX PDF manual that describes the equation editor support in Pages v6.1 and later.
These limitations mean that there is no means to use any packages such as amsmath that would allow forcing the asterisk into alignment. Even if you entered your equation in pure MathML, the asterisk would still be elevated. I know, because I tested that too.
In MacTeX 2018, which includes the amsmath package (and its smash operator) — this is the source and output when typesetting with LaTeX and pdftex: