Can a temporary worker file for workers compensation if they are injured? Are they covered by a companies policy?

Were they injured on the job?

I would note here that Texas is the ONLY state where the employer is not required to carry Worker's Comp Insurance. If they do not, a notice saying so must be posted where employees can reasonably be expected to see it.

Neither temp status nor the visa you hold has any bearing on whether or not you are covered under WC. As Patty pointed out, TX does not require employers have coverage (though most do). If they have a policy, then whether you have a visa or are a temp, if you are injured as a result of work, you are covered just like anyone else.

No one has been injured. Just wondering if they have access to file for WC if they are only here for 9 months and then must go back to Mexico. The company does have a WC policy.

To summarize:

If they are employees AND
If the company has Work Comp insurance AND
They are injured on-the job, THEN

Yes, they can file a work comp claim.

No one has been injured. Just wondering if they have access to file for WC if they are only here for 9 months and then must go back to Mexico. The company does have a WC policy.

Yes. They would still be covered under the claim, though it gets a bit more complicated once they actually leave the US. Their rights under WC are extraterritorial.