It would be rather unusual for watercooling blocks to be plated with or made of stainless steel, so it is most likely nickel although I'd think you could check what it is from the manufacturer's web page.
Otherwise, copper, brass, nickel, and to a lesser extent silver are effectively safe to mix because they only have small relative voltages on the
anodic index, which is how metal compatibility is determined. That means that any galvanic reaction will be small and slow assuming your loop is normal (undamaged blocks and plating, non-reactive coolant, etc.). Over the expected lifetime of a standard loop, using those metals, there shouldn't be enough galvanic action to matter (again, assuming that the loop is properly maintained). On the other hand, Aluminum has a more significant relative voltage when matched with those other metals which makes the reactions more vigorous, and thus cause damage a lot faster.
edit: I quickly checked out XSPC's specs on the Razor block and it's not clear from the description or photos whether the stainless steel plate gets wet or not, maybe someone who owns one can shed light on that.