Ewwww. Those look like some super bad calcium/gunk deposits. What kind of coolant was run before? It looks like it might have been Dex-Cool.
If a system running Dex-Cool isn't properly bled of air, then it can react with the coolant & create a snotlike gunk that will f up your passages & radiator. RMI-25 cleans that shit out like nothing. Google if it you need some.
Your problem might not be like this, but here's a case of an iron block & Dex-cool:
In any case, when it's time to fill up with coolant again, flush your system with some radiator cleaner & distilled water until it's clear, then fill with the OEM nissan stuff OR Zerex G-05 with some food coloring of your choice (due to the light color of G-05) & bleed your system well.