whats the red thing at the top of watch face with a diagonal line through it

Hi

The Disconnected status icon indicates that your Apple Watch and iPhone are not connected:

The status icons

If your Apple Watch and iPhone do not reconnect automatically when brought within range of each other, then follow the instructions here, if necessary including unpairing and re-pairing your Apple Watch - but not before having first tried the other recommended steps:

If your Apple Watch isn't connected or paired with your iPhone - Apple Support

Apple Support gave the following information:
Your Apple Watch isn't connected to your iPhone. Move your devices closer together or turn off Airplane Mode.

Only thing is, that same symbol has been showing up on my watch even when my phone and watch are on my person and neither is on Airplane Mode. Any help/advice?

Hi Phil

When your watch face shows a red phone symbol with a line through it, this means that your Apple Watch is disconnected from your iPhone:

Disconnections are normal and will happen whenever the devices are taken out of range of each other. After they disconnect, if they do not reconnect automatically shortly after being brought closer together again, follow these steps - checking the status of your watch after each one - until they do:

- Check that Bluetooth and Wi-Fi are both enabled on your iPhone and that your iPhone and Apple Watch are close by each other.

- Check also that you haven't enabled Airplane Mode on your watch (see step 4 for how to check this).

- Disable and then re-enable Bluetooth on your iPhone: (Settings > Bluetooth).

- Enable and disable Airplane Mode on your Apple Watch: Swipe up on the watch face > Swipe sideways to the Settings Glance > Tap the airplane icon to enable Airplane Mode (this turns off Bluetooth & Wi-Fi) > Wait for a short while > Tap the icon again to disable Airplane Mode (re-enables Bluetooth and Wi-Fi) > Connected should appear at the top of the screen.

- Restart both devices, turn both off first, then restart your iPhone before restarting your watch (to turn your watch off: press and hold the side button until you see the Power Off slider; drag the slider to turn off. To turn it on: press and hold the side button until you see the Apple logo).

For the best performance from your watch, keep both Wi-Fi and Bluetooth permanently enabled on your iPhone.

That is probably the alert, that the paired iPhone is out of reach.

See the list of Status Icons: https://help.apple.com/watch/#/apdce21b02a0