What is a Monoprint?
A monoprint is
frequently interchanged with the word "monotype". "Monoprint" tends to be used by academics versus those selling art to the design community who prefer the word "monotype".
An artist starts by "painting" or "drawing" with ink on a surface such as a printmaking plate made of aluminum or zinc. Some use a large pane of Plexiglas to paint on. The resulting image is transferred to a piece of paper by placing a sheet of paper on top of the inked plate, running it through a press. One monoprint is yielded looking like what the artist drew with ink. Some residue of ink remains on the printmaking plate. That faint residue of ink becomes the platform from which the artist builds his/her next composition. The secondary composition can employ different colors as well as compositional changes. An advantage of making monoprints is the creation of a rhythm of working, saving time by creating subsequent compositions using the "ghost image" as a starting point for the next work of art.
What is Foxing?
is a condition flaw on a work on paper.
Foxing yields a freckled appearance typically of red brown spots. Currently it is believed these are iron oxides and hydroxides produced by the chemical reaction between the organic acids discharged by mold and the colorless iron salts and impurities embedded in the paper.
Dampness encourages mold. Some types of sizing in the paper also attract fungus growth. Acidic contact materials such as cardboard and animals glues can aggravate the foxing.
Only a professional paper conservator can minimize the appearance of foxing safely.
To read more on this topic, I suggest The Care of Prints and Drawings by Margaret Holben Ellis as well as Prints and The Print Market by Theodore B. Donson.