Laundry Mystery Solved

I've been plagued for years by problems with my laundry. All my towels and bed linens are white, every so often I get faint yellow patched on things, sometimes quite marked yellow stains which I have to re-wash out.
I recently bought a quite expensive duvet/pillowcase set and within a couple of washes the pillowcases had gone a faded yellow.
Last week DH found all his towels had black/green marks on them.
In most of these cases a repeat wash (sometimes two) will get rid of them.
I've tried different powders, fabric softeners, no softener, but there just didn't seem to be any pattern.
Then this morning I pulled out the powder draw on one of my machines (which I clean regularly) and there was a thick, black lump of gunge in the draw. On closer inspection there is thick, black mold on the roof of the draw opening where the water sprays out. Both machines.
Clearly what's been happening is every once in a while a lump of gunge falls into the water and ends up staining the laundry. I think a major contributor to this problem is the fact we use eco friendly plant based washing powder so there's no chemicals to stop mold growth.
Cleaned it all out with a bottle brush, both machines, so hopefully no more laundry problems.

It's a known problem on some of the machines. You have to clean out the drawer regularly.