should a ww out sustain a nasus with executioner

How many stacks does Nasus have and how much CDR? If WW can get enough health back in between Nasus's Q, he'll probably out sustain (I forgot how Nasus's sustain work but I think WW is better in general) Nasus, especially as Nasus's health gets lower which increases WW's attack speed and therefore increases his sustain.

Also did Nasus have FH?

Like... Is Executioner's the only item nasus has? There isn't enough information here to tell. If it is just Executioner's, then yes, WW will out sustain that. Especially if it is a E maxing Nasus with 0 stacks. Executioner's doesn't cut off healing entirely, so like... there is a chance that if WW is ahead or in certain circumstances, that WW will have some decent healing.

Early game sure, later game when nasus should have a sheen item and 400+ stacks with a chockful of resistances, then no. Nasus should also kite in between Qs instead of standing still and autoing in between Qs.

Executioner's reduces healing, it doesn't negate it.