"Most likely not. Slime mold can definitely sustain short periods of observation under light. The slime mold plasmodium looks really white on that picture, sign of it's waste product traces as François-Joseph Deminiere's answer in…"
"Another form of "transparency" I've observed is when the slime mold travels around so fast that it only leaves slimy traces of waste products, as in this picture:
I'm not sure what conditions make it better for it to keep…"
This may be a pretty delayed answer, but I've found Physarum doesn't do well in watery conditions. Just yesterday I tried re-animating a small piece of dried sclerotium in a petri dish where I put too much water in, and…"