You probably are seeing amine blush which is a chemical reaction of the epoxy with humidity (i.e. water). You should be able to wash it off with soap and water. Scrub it with a green dishwashing scrubby.

> Just finished a stitch & glue (my second). Wanted to paddle during
> those weeks I'm advised to wait until the epoxy is getting fully cured
> before varnishing. Getting into the water the epoxy top layer became
> whitish. What did go wrong? What to do?