The short answer in 40 degrees Fahrenheit. The long answer is much more fun though so I will start by talking about freezing semi solids then move to refrigeration mechanics then move to thermodynamics within the refrigeration space. I promise to leave the math out of it.
The first things to freeze in your refrigerator are the thing high in sugar. Anything viscous jam, berries etc. Of course, you will also see your leafy greens wilt. Damage or ice crystals in these products is your warning. If you see them your first course of action should be to turn up the temperature in your fridge. If you continue to see the problem it could be a failed thermostat, fan switch, or a problem with how you setup your fridge.
Most refrigerators commercial or otherwise have their internal cooling unit on the top of the interior. This is no coincidence. We have all heard that heat rises and cold falls. That is absolutely true so refrigerators are designed with that in mind. The reason your leafy greens go in a bin on the bottom is because that is the area furthest from the coldest air coming directly from the coils. So if you are putting your leafy greens on the top shelf you are making a mistake.
The top shelf should be reserved for the items with the greatest heat load and density. I.E. warm leftovers, liquids, and casseroles. It is also important to understand the thermostat on a refrigerator usually operates on a differential rather than exact temperature. In other words the thermostat will wait until the refrigerator temperature changes 2 - 4 degrees Fahrenheit before initiating or terminating a new cooling cycle. So if your temperature is set to 36 degrees Fahrenheit you will likely have freezing temperatures in part of your unit. This can cause frost on the coils eventually causing unit failure.
When considering the importance of proximity to the coils we must also consider air circulation. All commercial refrigeration units and most residential units will have a fan to circulate air from the internal coil. Proximity and blockages should be considered when loading a refrigerator. The most common mistake when loading a commercial fridge is to remove all of the shelves and stuff it full of cardboard boxes full of product. 2 problems come into play in this scenario. The first problem by removing the shelves you have removed the gaps for air to circulate through the unit. Further, all commercial shelves will have a stopper in the rear to allow a gap for airflow. Do not remove, stack on top off or otherwise ignore this simple but important part of a refrigerators engineering. Secondly, cardboard boxes are great insulator. They can trap and block heat quite well. If the product inside a box is not already down to temperature do not load it into a fridge without at least opening the box.
So if you see frost consider all of what you have learned. If it persists despite your best efforts call RJH for assistance.