Legal Question in Real Estate Law in Texas

This is one that cannot be answered briefly. However, a good place to start is to read these articles:

http://recenter.tamu.edu/pdf/1152.pdf

http://recenter.tamu.edu/pdf/1334.pdf

Contact the owner in writing and notify them they do not have permission and they need to move the structure. Fences are a good decision. No trespassing signs help. All this is good but you need to check out your property on a regular basis to make sure storage sheds, houses etc are not put up. Also make sure they are not paying the taxes.

While the articles are old, they gives you a good foundation on the issues that you can build upon by doing a little more research.

It looks like the "squatter" is setting him/herself up to acquire your property via adverse possession. Time is not on your side, but it isn't too late to protect your interest. Please do so immediately.