Is there a formal language of Combinatory Logic's expressions?
The set of valid expressions in combinatory logic forms a computable formal language, and so can be described by a grammar. It is likely that this language is context-free, and so can be described by a context-free grammar.
More generally, we know the power of grammars exactly: a formal language can be described by a grammar if and only if it is recursively enumerable. So if there is an algorithm which enumerates all valid expressions in combinatory logic, then the language of all valid expressions in combinatory logic can be described by a grammar.
Even more generally, by definition, a formal language over an alphabet $\Sigma$ is just a collection of words over $\Sigma$. This means that any encoding of combinatory logic expressions which uses some fixed alphabet is automatically a formal language.