If WiFi Guard has detected a device, it means the device is connected already. It looks like in your case the device randomly changes its MAC address, which is common for mobile phones. Both iOS and Android devices randomise their MAC addresses for privacy reasons. So in your Samsung Galaxy phones the MAC address is by default randomised and this address is used instead of the device MAC address.
To turn this off on your Android phone, go to its
WiFi settings and tap the
gear icon next to the current network. Select
Advanced - Privacy and tap "Use device MAC".
On iPhone/iPad, open
Settings, then
WiFi, tap the
information 'i' icon next to your network, and then toggle "Use Private Address" off.
This change will only affect that particular network. Your device should continue using randomised addresses in other networks.
According to iOS and Android descriptions, on iDevices the randomised MAC address changes when the user 'forgets' the network, and every 24 hours if there is no active traffic-passing session at that moment. It stays consistent per SSID (per a particular network) on Android devices until they are reset or, as some users report, until the device is restarted.
This information about private/randomized WiFi MAC addresses was provided by Apple for their iOS 14 release:
Quote
apple.com
To further protect your privacy, iPhone, iPad, iPod touch or Apple Watch can use a different MAC address with each Wi-Fi network.
To communicate with a WiFi network, a device must identify itself to the network using a unique network address: media access control (MAC) address. If the device always uses the same MAC address across all networks, network operators and other parties can more easily correlate that address to the device's network activity and location over time, which allows user tracking or profiling.
To reduce this privacy risk, you can use a different MAC address for each WiFi network. This unique, static MAC address is your device's private Wi-Fi address for that network only.
Using a private Wi-Fi address doesn't affect how you join or use most WiFi networks. If your WiFi router is configured to notify you whenever a new device joins the network, you will be notified when your device first joins with a private address. If a network can't use a private address to provide parental controls or identify your device as authorised to join, you can stop using a private address with that network by disabling it in the settings. Rarely, a network might allow you to join with a private address, but won't allow internet access. If that happens, you can stop using a private address with that network.