I know it's possible to measure temperature using a laser, and Nexus 5x phone has laser-based focus. So it might be possible to use reading of the laser input and output to gauge temperature at what your camera is focusing on maybe via root access.
However, is it really possible to do that, perhaps using some apps?
I'm not an expert in this field, but reading the explanation of infrared thermometer on Wikipedia, I don't think general Android pdevice has sufficient component to do the reading. Here's the explanation from Wikipedia:
By knowing the amount of infrared energy emitted by the object and its emissivity, the object's temperature can often be determined....
The design essentially consists of a lens to focus the infrared thermal radiation on to a detector, which converts the radiant power to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature.
Even if the Android device has IR laser, I don't think it has a thermal radiation detector installed.
There's an app called IR Thermometer by Sensorcon on Play Store which needs their Sensordrone product to do the reading. Other product, like RYOBI Phone Works also claim that it can do infrared themometer, but again, you have to buy their sensor to do the reading.