You can make a paste of baking soda and water, apply the baking soda paste, scrub with an old toothbrush and let it sit for a short time, then flush with fresh water. You can also apply the paste to the battery cable clamp. Auto parts stores sell a battery post cleaning tool that can clean off a battery post and also includes a small wire brush to clean the inside of the battery cable clamp. Wear eye protection and rubber gloves as the battery acid residue is sulfuric acid based. Concentrated sulfuric acid is highly caustic to skin and clothing. The acid can be nuertalized by the chemical action of the baking soda and water. Repeat as necessary and flush clean with plenty of fresh water.
I would definitely send the photo to Tesla support and request it to be fixed, to at least make them aware of the issue (in case it damages something later).
^^^This. Did the tech know you took the photo of the tray? There's NO WAY the tech should have completed the job w/o properly cleaning the battery tray.
If the battery cage is plastic coated you can wipe it down and reinstall the battery. If it raw metal I would spray some paint on the metal before replacing the battery.
CRC makes a great battery cleaner and spray protectant. I’ve used it for years to help keep my batteries as healthy as they can be.