I've noticed it a lot from various translations and it's really minor, but on page 17 he would just say "scum", not "a scum". Scum is an uncountable noun (like "grass") so it is either used on its own, or with an adjective describing its form ("a pile of scum", "a puddle of scum", etc). Though using any of those honestly feels unnatural when calling someone scum, as it is primarily used on its own, or as "scumbag".
(Do note that, unlike "scum", "scumbag" would have a leading "a".)
Thanks for the tl!