I recently discovered a sort of green jelly/slime growing on a concrete path and foundations here, and a bit of digging has revealed this to be a plus, I think. I'm going to keep an eye out to see if the chickens eat it, in wet or desiccated (brown crust) form. Maybe you have some too and can contribute to the topic.
Its proper name is Nostoc commune, and it's a cyanobacterium with a penchant for alkaline surfaces (which concrete provides) and phosphorus (which chicken poo provides). And before that puts you off, note that it
(a) is edible, by us so presumably by chickens;
(b) fixes nitrogen from the atmosphere;
(c) has anti-inflammatory, anti-oxidant, and anti-bacterial properties, at least (and has long been a traditional medicine in some cultures);
(d) consists of about 25% dry weight proteins, peptides, and free amino acids, including all the essential ones that we have to eat because we can't synthesize via our own metabolism;
(e) may contain a variety of minerals such as calcium, iron and zinc, plus useful amounts of vitamin C.
It is also incredibly resilient (so difficult to eliminate, should you want to ignore its potential as a valuable food for you or your chickens). Aka Star Jelly, you can read more about it here
https://en.wikipedia.org/wiki/Nostoc