I missed your second question. Lol
If they are not being picked on, probably a molt.
Here's another link.
http://www.oznet.ksu.edu/library/lvstk2/mf2308.pdf
Imp
Could be perfectly normal.
I'll attach a link to lots of pix of poop.
http://www.chat.allotment.org.uk/index.php?topic=17568.0
Imp- Don't look before a meal.