It's hard to say from the blurriness of the picture, but it looks to be a skin abrasion, possibly caused by the algae eater (probably a pleco). Take the newt out and confirm or not if it is indeed a skin abrasion. If so, read this article and take the appropriate action:
http://www.caudata.org/cc/articles/sores.shtml
And Kaysie is right- seperate the algae eater from the newt.