It looks like a Tyrannochromis not a Nimbochromis, which tyrannochromis it is I am not sure. How large is it now? And is it showing any color yet?
Maybe this is the Polyodon, as my friend and I have the other 3 identified species of the Tyrannochromis swimming around, but they have a different pattern.
The body build and tear under the eye make me almost certain that it is a Tyrannochromis or a crosbreed with a tyrannochromis.
http://malawi.wimmels.com/english/index.htm Have a look under the Species tab and then at the tyrannochromis photo's