I have made yeast water many times and never had mold up until this time. I’m not sure if it was mold and even if it was it didn’t affect the performance. The only difference was that this time I used raisins that had oil on them and they made a skin like boiling milk does when they started floating on top. Perhaps that exasperated the issue. Other then that the raisins were floating, the mixture fizzing and smelled ok. It was just on top of the raisins where a very thin substance appeared on top like an ultra fine spiders web. So what I did was skim off the top and it all came off in one piece, discarded all the raisins and most of the yeast water, save for one tablespoon, and topped up with fresh water and raisins. My thoughts were if it behaves like yeast water then it is yeast water. Within 24 hours the raisins were all floating, the mixture bubbling and no mold. Just like a big refreshment using a little yeast water as inolculation. Looked and smelled very good. Made very good bread too.
Some others were reporting mold but not everyone. Some started again from scratch. I went along with if it looks like yeast water and behaves like yeast water then it is yeast water. Nothing off putting about the smell so no indication of anything bad. If it smelled off and looked more sinister then I would have started again but going along with the advice from Hamelman it was fine.