There are many names for it. Crap, sludge, muck, mulm, poo, gunk, mud, guck...
It's all the organic waste and old rotten food in your tank, it's unlikely to be directly killing anything but the volume of it is likely a symptom of what probably is killing them. Judging from just the pics that would be insufficient filtration and water changes plus probably overfeeding. Do some water changes, vacuum it out.
http://www.theaquariumwiki.com/Mulm