That was it. It wasn't quite latched, I latched it and originally it didnt work. I completely removed it and reengaged it and the car started. I still have the icon on my screen but I will see if it goes away.
After you insert the orange traction battery interlock, you need to pivot the lever 90 degrees. Then you need to slide it down to lock in place.
X2 Neither of those residues look like leaking batteries but you can just mix some of it up in a pool water tester to see what the PH is if you're concerned at all. Dried up KOH from a module leak looks like an alkaline battery that burst (we've all seen the white powder when that happens).
To the OP it looks like alot of the metal in the back hatch is corroded too like the spare tire rim etc which is pretty good evidence that maybe you have a water leak (roof) also or were flooded somewhere. Take the spare tire out and check for standing water corrosion under the tire and with flashlight look deep in the 12 volt battery well for evidence of standing water. You may have bought a salvaged car that was flooded. Hundreds of salvaged swamped Prius out there. They seem to get totaled by I companies quicker than any other car. Whats the VIN look like?
The VIN is good, AutoCheck came back clean. I did notice some of the same residue in the spare tire area as well. In retrospect it was definitely pooling in some of the indented areas of the trunk metal.