[Cearbhaill]

Thanks everyone- I do believe you/we/I have solved this one.
It was the lighting.

And here I thought I was doing so well by using a marine specific bulb- I was using a Coral Life 50/50 purchased especially for my refugium.

I put a small table lamp beside the fuge every evening, and I now have green chaeto as opposed to the greyish melting crap I had previously. There's precious little left, but at least I resolved the issue and feel oh so much better about buying more!

So incandescent lighting did it for me.
I am doing the happy dance!