This is a phototransistor. It has a collector (short prong) and an emitter. Tested the unit with a 10K pulldown resistor connected to the emitter and 2 AA batteries to the collector. I got readings between 0.5V and 2.4V with indoor light.
This type of photoresistor is made of environment-friendly materials(no Cadmium and Lead) compared with the traditional CDS photoresistor.
It changes resistance depending on the amount of light it is exposed to. These little sensors can be used in light-interacting projects.