Alexandru,
The current flowing in the input terminal is the input bias current of the op amp. This current is normal with the OPA1662, a bipolar (BJT) input op amp. This current is relatively constant with changing input voltage so the input impedance is still very high. For an explanation of the difference between input bias current and input impedance you can check out this link:
“I Need High Input Impedance!” input impedance vs. input bias current
You should select a CMOS or JFET op amp for low input bias current in this type of application. The OPA171 (or OPA2171 dual) would probably be a good choice.
Regards, Bruce.