First, note that it is common for devices to have multiple IPv6 addresses.
You can list all of them associated to your Pi-hole's eth0 interface with
ip -6 addr show eth0
Yours above is a public IPv6 address (as identified by fitting the global unicast range 2003::/3) and as such, its ISP provided prefix might change when your router's IP address gets renewed (happens customarily, e.g. once every 24 hours for private lines in Germany). And you don't want your Pi-holes address to change at all.
Using the non-routable link-local IPv6 address (as identified by fitting the fe80::/10 range) would suffice in a simple network where all devices are on the same network segment (so no clients behind additional routers, access points, L3 switches,...), so could well fit your needs.
If your router supports it, consider configuring an IPv6 ULA prefix (for the fd00::/8 range) that will be picked up by your devices (including Pi-hole), is visible throughout your home network and won't change (unless you change it in your router, of course).
Read more about this at Use IPv6 ULA addresses for Pi-hole.