I had a biopsy on May 15. It came back as squamos. I am having Mohs on Aug. 29. The spot is growing back. Should I be concerned?

Squamous Cell carcinoma like you've been diagnosed with is the more aggressive of the 2 common skin cancers. I may try to get that resection date moved up if possible. Also, regarding the closure, it's always a good idea to evaluate how the MOHs surgeon performs closures and to see if they have before and after photos that you can look at. 
Hope this helps and God Bless!
Dr. Robb

A biopsied skin cancer that resurfaces before definitive treatment is common and is evidence that the cancer is still present. However, some squamous cell carcinomas can be aggressive and grow quickly. If this seems to be the case with your skin cancer, I would suggest moving your Mohs appointment to an earlier date. The smaller the skin cancer is when it is treated, the better your surgical outcome.

The purpose of a biopsy is only to identify the lesion, in this case it's squamous cell carcinoma. It is not uncommon to see some regrowth of a lesion after a biopsy. In the case of squamous cell carcinoma, my concern is that your surgery is so far in the future. While skin cancers like basal cell and squamous cell carcinomas are usually slow growing, the fact that some of this has grown back is a bit concerning. Check with your Mohs surgeon for his/her thoughts on your particular lesion and express your concern. There are many fellowship-trained Mohs surgeons in your area if you prefer not to wait.