It would typically indicate a memory leak in the nsids binary. If it runs as a service then a restart via cron once or twice a day may keep it from crashing the device, but working with the developers to track down the leak would be a longer-term approach.
EDIT: I see the source code is on github. A half-decent developer should be able to track down the cause of memory usage by instrumenting the code, using valgrind, etc.