If you communicate with your crystals, for example using a quartz crystal to facilitate your intuitive outreach, can interlopers exploit this to connect to you and cause harm?

This can happen in the same way any outreach of thought can be answered. This is why, in reaching out beyond the self, it is important to establish protection first, and to safeguard the process to be undertaken. This is no less true of doing an intuitive outreach facilitated by quartz crystals. If there are no constraints, other beings, other forms of consciousness, can use the same crystal to hop on your wavelength, so to speak, and return your thought with thoughts of their own. The ideal is to combine the practice of using quartz crystals as a facilitating device, with the instruction to maintain safeguards and to request divine protection in the event of any intrusion. And this will do the job nicely to maintain safety and allow the implementation of these crystals as a useful amplification of intuition.