what to use as mulch for potatoes

I think the problem with using stuff that is not fully composted is that it attracts a lot of slugs and they can ruin your crop, although that can also be a problem using straw.  John who owns the site has a couple of articles you might find helpful 

A deep layer of grass clippings might go a bit wet and slimy, but I know some people on here use them.  I think that may be for potatoes actually planted into the soil and the clippings are instead of earthing up, but someone will hopefully be along at some point to clarify that.

https://www.allotment-garden.org/vegetable/potatoes/growing-potatoes-under-straw-mulch/

https://www.allotment-garden.org/vegetable/potatoes/growing-potatoes-under-black-plastic/

As an experiment, I earthed up a tub of potatoes a year or 2 ago with grass clippings(a few inches of compost at the bottom). We got some good potatoes in the end from it