Your current hitpoints cannot exceed your maximum hitpoints.
The important clause from feign death is:
the disease and poison have no effect until the spell ends.
So the effects of the disease are only temporarily suppressed by feign death and if feign death ends before harm, the effects of harm return. In particular, your hit point maximum is reduced to ...
The target is most likely unconscious until the end of their next turn
The feature in question states:
[...] a creature within 30 feet that looks at the prismatic beetle swarm must make a successful DC 13 Wisdom saving throw or be blinded until the end of its next turn. If the saving throw fails by 5 or more, the target is also knocked unconscious. [...]
Half Orcs don't have anything to increase HP (besides a con bonus), fighters have d10 hit die. 18 con gives you +4 con bonus, so that increases your hp by 4. So at level 1 you have 10 + 4 = 14 max hp.
If they were level 2, and yet to roll hp, they would have 18/18 hp. They could have also accidentally changed their hp manually or added feats.