Chromecast Multicast Between SSID Subnet VLAN On Fortinet Fortigate

Enable Multicast Policy:
System -> Feature Visibility -> Additional Features > Multicast Policy -> On

Configure Multicast Policy:
Policy & Objects -> Multicast Policy

You'll need to create a policy going both ways between the desired networks and ensure that SNAT is enabled as well.

You may also need to disable multicast ttl changes via the CLI:

config system settings
set multicast-ttl-notchange enable
end

FortiWifi, 60D, FortiOS 6