regarding avalon interface

It doesn't matter whether it's an actual memory or not. Every slave component has at least one addressable location (the "base address" you set in Qsys) that is used for data transfer (like a memory) or for reading or writing control or status registers. I don't know what your sequence detector does (custom IP?), but you access it from the Nios either with a hardware address or using the macros created through the use of the Nios software build tools (the system.h header created based off of the Qsys-generated .sopcinfo file).