Cleaned off the bottom board 8 hours ago. New beekeeper. Nuc only installed 1 week ago
The large bits are chalk brood mummies. I’m not sure what the blackish gritty stuff is. The lighter colored stuff is most likely brood cappings created while young bees emerge.
The black bits might be mildewed wax. Your hive might need some help with ventilation.
I was thinking the black bits were some sort of frass.