How to get to Torch on Moto G / Marshmallow 6.0.1
This appears to be an old problem cited two years ago wherein torch light and cast icons as in case of OP go missing. Reason attributed is some icons disappear if not used for long and reboot brings them back as it happened in OP's case.

Probably this is a bug that is continuing and not found in latter models - I don't see that on Moto X Play (also maybe because I use  Gravity box Xposed module interface for quick settings)

It sounds stupid, but it looks like you have to remember to use those icons to be able to find them when you need them :(