Black mould on window sealants - or is it?

[JohnnyMcGrathSaysFuckOff]

We have fairly standard white upvc windows, with white upvc frame > black sealant strip > glass pane.

When I clean the windows, the black sealant strip seems to be covered in black mould and leaves long black streaks on the cloth.

But I clean the windows weekly. Surely not that much mould can be building up week to week?

Is this actually colour transfer from the sealant?

Or, is it mould - and how to get rid once and for all?

If you don’t ever have the windows open then that strip can, when rubbed with a light cloth, show dark streaks

We have fitted new up clothes windows in our last 2 houses.

My bedroom, bathroom and living room windows don’t always show dark (but do sometimes) but DC who never opens their window does. Its due to constantly closed windows dripping condensation down so meaning the seal is always wet so colour is more transferable ime.

Hi, we do have those windows open every day! But I do wipe the seals with a damp cloth so maybe it is just colour transfer?