Normal hedgehog droppings can range from almost pellet-like to quite soft and
sticky.  Colour is usually very dark brown, almost black.  Depending on diet,
especially treats, they can vary quite a bit.  If your hedgehog is leaving
unusual droppings after having had a treat or change in diet a day or so
before, then it is probably related to what he ate.  If the problem continues
(assuming the hedgehog is back on his normal diet), or if your hedgehog is
suffering from severe diarrhea, see your vet, immediately.

One thing that can help solve diarrhea is live cultured yogurt:

    I have received some info from my vet about diarrhea in hedgies.  
    He said that one of the best ways to cure it was to feed yogurt 
    with Lacto Bascillus such as Alta Dena.  He also said that most 
    hedgies LOVE pina colada flavor (and mine do).
    -- Ian Van Natter

There are some serious problems that are indicated by funny looking stools.
Remember that if you've fed your hedgie something odd, that is likely the
cause of the problem, but if he's been on his normal diet, and changes in his
stools happen, it may be a warning sign.  For example:

    A good sign that there's something definitely wrong with your hedgie 
    is stinky black tarry looking stool.  If anyone's hedgie shows that 
    it's [potentially] an internal lesion and the hedgie should be rushed 
    to the vet IMMEDIATELY.

    The vet said he had necropsis of the kidney, mineralization of the 
    heart, a lesion in his lower stomach and a touch of pneumonia or the 
    flu (they think he got that the last day he lived, just another 
    infection setting in).  They think that it was his kidney's malfunctions 
    that brought that all about, and they think that it was something he may 
    have been born with.

    -- Ligia Ortega (via her veterinarian, from tests following the
       death of her hedgehog, Howard)

While similar symptoms in your hedgehog might not indicate the same problem,
a trip to a knowledgeable vet is probably worthwhile.  In general, serious
kidney problems in hedgehogs are probably not easily treatable, but catching
such problems early might make a big difference.

Forest green stools also indicate a likely problem. Often these are
indicitive of some sort of internal infection.  A visit to the vet is usually
required.