If someone knows my passcode can they erase my phone?

This support article has a list of things to do if your iPhone, iPad, or iPod touch is lost or stolen. --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Depending upon your settings they might be able to erase it with your passcode alone but if you had Find My turned on they would need more than the passcode.

"If you turned on Find My [device], you might need to enter your Apple ID and password. "

from What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If the person knows your phone’s passcode, for nearly all functions they ARE you.

Hello ~ Have you received any messages saying they found your iPhone and you provided a response with any information?

~Katana-San~