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This is surely a textbook case of 'water off a duck's back.'
Literal
Exactly frog [of-の] face [on-に] water [expression-とは] this thing [probably-だろう].
蛙の面に水 'water on a frog's face' is a ことわざ (proverb) expressing that something has no effect on the target — equivalent to English 'water off a duck's back.' The pattern [expression]とはXだ 'what is meant by X is Y / X is exactly Y' is used to identify a situation as a textbook case of the stated expression. まさに 'exactly, truly' emphasizes the perfect fit. The sentence assumes the listener knows the proverb and is labeling a just-observed situation with it.