Buck Converter Feedback Circuitry
To add another specific to what The Photon said, error amplifiers in switching regulators are often implemented as gm amps.  So they take the difference in voltage on the input and convert it to a current with a gain of gm (A/V).

That's a really easy way for IC designers to save a pin as the compensation network goes between the output of the gm amp and ground, converting the current back into a voltage that goes to the modulator. The design of a gm amp is easy too, but the downside is that the gm usually varies 20-30% from part to part making the end user's job harder, typically requiring more gain margin.

The symbol is sometimes drawn with linked rings on the output, or without the point as shown below, but it's sometimes drawn like a regular op-amp.

From Here:



For the circuit shown in your question, it could be that the symbol is a block that encompasses the feedback and compensation, or it could be an op-amp compensator where the user must put a feedback network between the comp pin and the feedback pin. (Also somewhat common).

You can see that here: