Hi and welcome. I think it is poo too and your goldfish isn't well. How long have you had the fish? It could be just settling in (they can sulk for a while) and, depending on what it was being fed, it can alter the colour and texture of the poo. Bits of red in it, plus it being white and stringy, doesn't sound good and it might have internal parasites of some sort (inc worms).
Any sign of redness about the gills or breathing rapidly?