In CoS, Ron famously casts the spell to Malfoy saying 'eat slugs', but it backfires and he ends up on the receiving end of it from his faulty wand. It got me wondering - is 'eat slugs' an actual incantation, or was Ron using a non-verbal spell at the time (possible because he was so angry perhaps), or was it something else I'm not considering?
Yes it is. In the book "Order of the Phoenix", it states that the slug-vomiting charm is a recognised spell.
"Harry was reminded forcibly of the time that Ron had accidentally put a Slug-Vomiting Charm on himself."
In the film, Ron shouts "eat slugs" whereas in the book, he simply casts it nonverbally.
A loud bang echoed around the stadium and a jet of green light shot out of the wrong end of Ron's wand, hitting him in the stomach and sending him reeling backwards on to the grass
Moving down the canon order, the spell is explicitly named in Lego Harry Potter Years 1-4. You can buy the Slugulus Eructo charm from Wiseacre's Spells.