Google and Adobe, who are jointly involved in one of the largest co-ordinated efforts to create a pan-Unicode font family - the Noto fonts mentioned in your question - use "tofu" or "tofu-like blank" to refer to a glyph with a rectangular outline reminiscent of a block of tofu.
When text is rendered by a computer, sometimes characters are
displayed as "tofu". They are little boxes to indicate your device
doesn’t have a font to display the text. (Source)
[My] long time manager, David Lemon, is the person who originally
designed this glyph, which was meant to convey a clearer “glyph not
available” message than a blank or tofu-like shape. (Source)
MediaWiki developer Santosh Thottingal has helpfully defined "tofu" in relation to the OpenType specification:
What is tofu?
It is a glyph inside fonts with the glyph name .notdef as per Opentype
says "The .notdef glyph is very important for providing the user
feedback that a glyph is not found in the font. This glyph should not
be left without an outline as the user will only see what looks like a
space if a glyph is missing and not be aware of the active font's
limitation. It is recommended that the shape of the .notdef glyph be
either an empty rectangle, a rectangle with a question mark inside of
it, or a rectangle with an “X”. Creative shapes, like swirls or other
symbols, may not be recognized by users as indicating that a glyph is
missing from the font and is not being displayed at that location."