After reflection, I'm inclined to see "the Good" as an evaluative statement that expresses a measurement of the magnitude of pleasure. On a scale of 0% to 100%, we might describe "the highest good" as those actions which most reliably facilitate the cultivation of maximum pleasure. Therein, "the Good" is not necessarily pleasure, itself (since pleasure is elsewhere defined as the goal of life), but rather, an evaluation of the means by which that goal is achieved.
Contrasting Epicurus "good" with Plato's might be helpful. Plato's Form of the Good reads to me like a contemporary description of God the Father (I am reminded that C. S. Lewis ends his Chronicles of Narnia with a character, having been resurrected, exclaiming "It's all in Plato, all in Plato.") The Form of the Good is supreme, existing beyond space and time, the origin of knowledge against which all forms can be compared to define their identity and agency.
This sort of a priori, transcendental knowledge provides a juxtaposition against Epicurus' preconceptions. Whereas the preconceptions are mental impressions that come from nature, the Form of the Good is the foundation of reality from which nature gains its (lessened) identity. The Form of the Good is the only thing that can be said to truly exist; the identities of the daily forms we experience are defined according to their relationship with the Form of the Good.
Contrary to the descriptions from the scholars I cited earlier, pleasure cannot be the only good, because Epicurus directly identifies prudence as the "highest good", as well as comparing "goods" to "virtues", so that leads me to believe that Epicurus recognized a host of goods, each of which can be measured against the others according to which one most reliably and successfully provides the means by which the goal of life (which is definitely pleasure) is achieved.
... but he does also identify pleasure as another good, and pleasure is definitely not a virtue.
It may behoove us to distinguish between "good" things, like dogs, sunshine, and pleasure versus "the Good(s)", a category of natural virtues which include prudence and wisdom. The adjectival employment of "good" is used by Epicurus as a functional descriptor to express approval; it is also used as a noun in reference to (as I read it) an evaluation of the efficacy of an action to produce the goal of pleasure.