I had that on my oldest stripper along area in the cockpit on the heaver glass tape. I think as you guess, the epoxy has vanished and I think salt has logged itself in micro pinholes in the epoxy and or glass. I think it's salt and I know you say you do mostly fresh water. Or its a chemical type of salt that gets produced from water contact with the epoxy and glass fiber traps it? My original OI where this occurred had no UV protection inside the cockpit under the deck. And thats where it was happening. Maybe your varnish was doing a good job protecting the epoxy ??

Anyhow, I know you can wire brush it off. That's what I did. Then I put a light coat of epoxy on it and later a coat of UV poly varnish. I know from being a boater for many years, sea salt gets hard and you can no longer just wash it off with water like when it's fresh. I don't know what happens to it but it doesn't not easily dissolve and that's what it seems like to me.