Birds and the bees
To be entirely honest, the white streak didn't look bad. In fact, it looked quite cool. The reason Nero was able to stay so calm despite the sudden change within his eye was because this was something he had already anticipated, although admittedly it had slipped his mind while he focused on testing his new innate abilities' limits.
His eyes and hair had turned blue because of the elemental affinity of his cryoflame, which appeared as a blue flame. By that logic, it was completely reasonable for his eye colour to change to the colour corresponding to his second ability.
What was interesting, however, was that instead of silver, like the moonlight, the streak in his eye was white.
Internally, Nero changed his ability back to cryoflame, and then summoned a tiny flame on his finger to see if the white streak disappeared. Unfortunately, the change was not immediate. It would take some time.