Likely the body trying to clear toxins through the pathway of last
resort.  The skin isn't very efficient for the purpose, but the body
will try to use it when other detox pathways are overloaded.  May be
exacerbated by malnutrition and/or poor hygiene.

If it looks more like a rash, it might be an allergic reaction. This
can be very serious because it also affects the airways and other
mucous membranes and organs.

I don't know that it has a special name or appears different from
any other acne or rash.  The cause, in general, is the same.  I
haven't been able to bring up anything special in searches.  You
could try a search for "acne lesion" or "skin rash" at
images.google.com for general pictures.

Carol

Is it crack cocaine or meth?

If it's crack it could be a lack of hygine. If it's meth then it's
probably the "meth bugs" and it's not acne. High levels of dopamine
cause a person to feel as if bugs are under thier skin and they will
scratch insessantly. Google for the "Faces of meth".