You will notice that the cooking time for this jelly is very short, thanks to the agar-agar
which acts very rapidly to jellify the juice. Contrary to a commonly-held belief, it is not good to cook jams or jellies for a long time. The longer the cooking, the less fruit flavour and vitamins remain. There's no need to fear "artificial additives" in this case, as agar-agar is a natural seaweed extract, which these days allows use to produce rapidly-cooked jellies with all the flavour of the fruit.
It's not absolutely necessary to skim off the foam when boiling, but this is a part of the jelly that has less flavour and a less smooth texture, which will form an unattractive layer in the top of the jars if left.
Perhaps this foam evokes in you the same reaction as in me, strong as some personal Proustian madeleine, as it brings back memories of my grandmother Jeanne making jam at Bonneveaux when I was about ten years old.