Don't know enough to know that the current name is optimal, but at least "Monoid" correctly says to the initiated "you don't understand this yet." "Appendable" lies to you and says "you understand this" even if you don't.
"Monoid" also has the advantage of being very searchable. First result on Google (from a private browser window) is the Wikipedia page with the snippet "In abstract algebra, a branch of mathematics, a monoid is an algebraic structure with a single associative binary operation and an identity element". DDG gives similar results, though the Doctor Who aliens are higher up...