Strange layer on top of apple cider vinegar while fermenting
I believe this is similar to the scum you see when fermenting pickles and other veggies. In a publication from the University of Georgia, it is noted that:
“While vinegar is fermenting, a jelly-like layer forms on top that is called the “mother of vinegar.” The mother is trapped and removed during straining. It can be carefully saved and refrigerated for use as a starter with your next batch of vinegar”
Just FYI - I spoke with a food scientist in our department and he advised that you also need to watch for mold, especially if there is a lot f sugar remaining. He doesn't think that is the case here, however.
What you have can be strained out.