Short answer: it is a false positive. I don't know exactly what causes it but I would guess Apple's Bonjour protocol, which is why you see something every 30 minutes. That's just a blind guess, but seems to fit.
Realize that a report of ARP poisoning wouldn't be likely on a private LAN, unless you got infected somehow. No known malware like this for iOS devices (and much harder to insert one on AppleTV versus an iPhone or iPad.) There are legitimate cases where ARP spoofing is used. And even Cisco has instances where they say to ignore that warning:
CSCsm25943—The meaning of the following error message on the controller is not clear. This message does not necessarily imply that any actual "ARP poisoning" is occurring. Rather, this message appears when a WLAN is configured for DHCP Required and a client (after associating to this WLAN) transmits an ARP message without first using DHCP. The client is unable to send or receive any data traffic until it performs DHCP through the controller.
DTL-1-ARP_POISON_DETECTED: STA [00:01:02:0e:54:c4, 0.0.0.0] ARP (op 1) received with
invalid SPA 192.168.1.152/TPA 192.168.0.206
Workaround: Perform the following steps:
...
• Verify that the client eventually does perform DHCP without undergoing an unacceptable outage. If the outage before performing DHCP is acceptable, then you can ignore this message.
I'm not saying that Norton's message is the same as Cisco's. Just that Cisco states that the meaning of why the message appears is not clear and sometimes is acceptable. And Cisco is the world leader in networking technology so if they don't always know why you get an ARP poisoning warning.... 😉
I won't go into the politics of "Norton bad" or whatever, but based on my experience (bias) with Norton in it's various forms for over 10 years, IMHO you can ignore this. Hopefully you can configure Norton to selectively ignore this. If not, you may have to use a different security program. Me personally, I do not recommend any "security suites" because they cause exactly this kind of additional headache. Just a "plain" antivirus program. Windows has a built-in firewall and most people will be using a hardware firewall at the office or home so the firewall in the "security suite" is extraneous.