It is perfectly correct and grammatical. "genius" as a concept (of brilliance, inventiveness, etc.) predates genius as a person (one gifted with genius). And one of the early meanings of "to answer" is to rise up to the challenge.
The simple answer is, yes. This usage of 'genius' as an adjective is relatively new and would still be considered by many (including me) as slang. In that vein, the following would also be acceptable: my genius girlfriend Your plan is genius! However, it would be more standard (and advised in formal settings) to use the word 'ingenious' in all of these examples, including your own, as it is an ...