Could you upload a picture of the issue?
If is a pixel issue i suggest you to read the warranty information for your country using the following link
http://www.sonymobile.com/gb/support/phones/xperia-sp/support-documents/
At page 14 of UK warranty you can read that
There may be tiny bright or dark dots on the display. It occurs when individual dots have malfunctioned and can not be adjusted. Two defective pixels are deemed acceptable.
Probably if the pixels are only 2 or less Sony will not consider it an issue.
So i suggest to go to a sony repair center to verify how many pixels are defective and if is possible to replace LCD panel in warranty.
