Solved What is this stuff

I'd guess it is some sort of fungus. When you say they are clearing it away, how are they doing that? Are they washing the area with bleach or anything? Are there any trees/shrubs nearby that could be dropping spores or something?

 

Very first thing I would be doing then, providing that there isn't any plants etc that would suffer is to brush it away and then get some bleach down there and give it a good scrub with a brush before rinsing off thoroughly. The alternative is Jeyes Fluid, but that costs an arm and a leg nowadays

 

There must either be something being introduced into the area or which is already there that is decomposing as Fungi need something like that, for whatever period, to thrive, as their action is to feed off decomposing fluid or material. Jeyes Fluid will help but not if the material is periodically being introduced from somewhere.