‘White dust’ underside Chard/Silverbeet leaves

Hoping to find an answer to this mystery going on with the chard (in Australia called silverbeet). I noticed a light white dusting on the underside of the outer leaves of one of the chard plants. Two plants next to each other, one affected, the other one not. At first I thought it was powdery mildew but I’m not entirely convinced to be honest. I know what powdery mildew looks like from zucchini and cucumber and usually the leaves are covered all over but not like this, not just on the underside and nowhere else. Also the fact that the other plant doesn’t have anything makes me doubt my diagnoses but then again, it might be just a matter of time…
It doesn’t look too bad when dry but when I put it in a bowl with water it looks spectacular. Like a spider has been busy making webs all over! Looks beautiful, almost silver. When taken out of the water, it is still visible but not the same as when in water (great trick of light!). It does come off when rubbing it but not with just running water. So, 1. What is it? 2. Can I still eat it (stir fried with some onion and garlic, yum!) 3. If not eating it, then probably better keep out of composting bin?
Thanks!

I've experienced this powdery white stuff on chard, but it was on the tops, not the underside. It was the beginning of powdery mildew spread by insects-first appeared as tiny white lumps where the injury/infestation started. Cut it back and hit the remaining leaves with neem oil or spray the leaves with a milk and baking soda mixture.

This is a new one for me.

I first thought it looked like snail slime, or insect webbing. Maybe try what Mark does.

Thanks to all of you who replied. It is still a mystery. Maybe it is mildew but the fact that it’s only on the underside of the leaves makes me doubtful. Also considered the spider mite but it’s not web-like.  A good soak in water with vinegar was enough to get clean leaves. First followed the sensible advice and cut it back but I just noticed something developing on both sides of one leave so will take it out completely, not taking any risks. It might be mildew after all. We’re only at the start of the growing season and on the other side of the gate I have cucumbers and zucchinis, don’t want them affected this early in the season!
Thank you all, really nice to be on this forum!