Nios ii ip can run on the xilinx fpga?

For a normal user, the NIOS IP core comes as encrypted library. Only Quartus knows how to read these files. Quartus only compiles for Altera devices.

In theory, you could buy a source code license, which will cost you some thousands of $.

Then I see no reason why it should not be possible to compile it for Xilinx.

But for the same effort, you could also use the Xilinx processor core. I'm certain they have something similar, which integrates better to the Xilinx tools.