You need an unspliced #12 from the j box back to the panel 680.23(F)(2)Through-wall lighting assemblies, wet-niche, dry-niche, or no-niche luminaires (lighting fixtures) shall be connected to an insulated copper equipment grounding conductor installed with the circuit conductors. The equipment grounding conductor shall be installed without joint or splice except as permitted in (a)<anpther listed light j box> and (b)<grounding terminals in a switch box that controls the light>. The equipment grounding conductor shall be sized in accordance with Table 250.122 but shall not be smaller than 12 AWG.
He can't splice in a receptacle box or any other box that isn't for switching the light!
As for the ground to the forming shell <in addition to the EGC in the cord> it is either a brass conduit or plastic. If plastic 680.23(B)(2)(b)Nonmetallic Conduit. Where a nonmetallic conduit is used, an 8 AWG insulated solid or stranded copper equipment grounding conductor shall be installed in this conduit unless a listed low-voltage lighting system not requiring grounding is used. The equipment grounding conductor shall be terminated in the forming shell, junction box or transformer enclosure, or ground-fault circuit-interrupter enclosure. The termination of the 8 AWG equipment grounding conductor in the forming shell shall be covered with, or encapsulated in, a listed potting compound to protect the connection from the possible deteriorating effect of pool water.
Note that can be stranded, the ONLY place I know of that specifically allows stranded #8, although some people attempt to make the argument #8 stranded is "larger" than #8 solid thus OK. I point to 680.26(C) which is pretty unambiguous "(C) Common Bonding Grid. The parts specified in 680.26(B) shall be connected to a common bonding grid with a solid copper conductor, insulated, covered, or bare, not smaller than 8 AWG."