I don't have an iPhone nor do I use GR-devices, but I have something similar going on with my wifi.
When I check my Network, I can see a device, usualy a Smartphone I don't know. That device was never connected to my wifi, first time I saw that device I was worried. But after some research I figured it is a device that has "Allways connect to WIFI when in range" enabled. So the device tries to connect to my wifi, the router notes the connection but due to the missing password there is no real connection going on.
I added MAC-blocking but it never solved the problem, the device was or is visible sometimes. To block devices on Meraki Go: Blocking Devices on Your Meraki Go Network - Cisco Meraki
I assume what you see is a device that did try to connect to your wifi but got blocked. You could ask someone you know with an iPhone to come by and see what happens (don't tell them the password).
If it shows the same result, you can be sure your wifi is secure.