Red pond water

I've got a small wildlife pond which I built back in the autumn, I recently added some hornwort and a lily from my other pond, however the water has turned red.

The hornwort is dying off, and I have noticed this week that there is also a red coating to some large pebbles aswell.

Any ideas folks as to what's causing this?

is the pond clay lined or pond lined it sounds like something is leaching ointo the water column are you sure that the water is red and not rusty orangy red