We're living on a planet whose name (in most languages, including both English and ancient Greek) is synonymous with "dirt", "rock" or "soil". Which, of course, is quite reasonable and descriptive, but it's bound to introduce some ambiguity in those rare instances when we're actually talking about the soil of another big ball of rock.
Anyway, the point is that, on both etymological and practical grounds, the word "geology" can be reasonably interpreted either as "study of the planet Earth" or simply as "study of the ground". In most practical contexts, both in the past, the present and the near future, those are effectively the same thing anyway. It's only in edge cases like "Martian geology" that one interpretation makes more sense than the other.
In fact, even without considering other planets, there is an argument to be made for favoring the "study of ground" interpretation: traditionally, study of the Earth's atmosphere, hydrosphere and biosphere are not considered to be within the domain of geology, even though all those things are also part of the planet. Of course, being part of the same planet, they do interact, and so a geologist might indeed care quite a lot about, say, wind erosion or lake sedimentation or coral reefs, but only insofar as those affect their main subject of study, i.e. the ground.
So, given that Mars is also a rocky planet like the Earth with a similar (though, also, in many important and interesting ways different) lithosphere, I would say that "Martian geology" is a perfectly reasonable name for its study. Especially so since the methods and principles involved in studying it are very much the same as here on Earth, and the results are likely to shed light on the internal structure and history of both planets.