Black slime in cistern

[Vicdiy]

My neighbour has a Siamp built in toilet cistern. He has a problem in that in the winter months he gets black slime building up in the cistern and coating the flush and inlet valves. He only has one toilet so no others to compare it with. Myself and his other neighbours with normal exposed cisterns never have this problem. Somebody told him it was ferrous compounds in the water but this doesn’t sound likely and even if it was surely we all would suffer the same problem.
Does anybody have any ideas as to what could be causing his problem and why only in the winter months?

 

[WC1]

In the zone where splashing occurs warmth encourages aspergillus mould to grow. Some cistern more susceptible than others probably for environmental reasons. Not dangerous imo but don't eat it or snort it.

 

[Ric2013]

I'm going to guess that there is a difference between the surface temperatures in concealed and exposed cisterns (also ceramic vs plastic) and, in this context, that means his cistern experiences more condensation and hence better living conditions for slimy things.

Since he won't be drinking his toilet water and generally will not see the insides of the cistern, I would not personally classify this as a problem.

 

[WC1]

I think concealed have less airflow around them so may be more susceptible.