Stuck clock

As NIOS sees you are adressing the memory in 16 bit mode, it will not toggle the lowest address line.

Most memories have a means of switching between 8bit and 16bit access, where one would need the lowest address line. That's why it is created whatsoever.

Your memory is fixed to 16bit, so the lowest address line (sometimes they even call it A-1) seems to be omitted already on the device.

From that, I would think you might need another address line (a nineteenth one).

i would think, with 18 address lines the nios system allocates only 256kb of space, not 512kb; you should check it in the address map.

Edited: Forget this sentence. As you are conecting the SRAM directly to the bus, it will not show up in the memory map.

With that in mind, you can safely ignore warnings about the stuck A0 line and try accessing the device. Be prepared to try and fiddle with the address lines a bit before you get valid results. Maybe you even need to change the code for the device. I had these problems when getting a parallel flash to work, which uses the same interface.