Ive just noticed that my black axolotl has a pearly white eyeball? does anyone know what this could be? =(?

He either scd it against something or it is pop eye, a fungal infection in the eye. It is usually caused from bad water quality. Check your levels. You can best treat it by water changes until your levels are good again. And it usually heals itself. If it is not gone in a couple of weeks then treat with an anti fungal like methylene blue or pimafix. Same goes if it is an injury as well. I had a fish with pop eye once and it healed on it's own as soon as I got my nitrates under control, and the fish did not lose his sight. It happens to the best of us, just keep an eye on him, he should be fine.