I think I've read somewhere in the forums that grass seed from any home improvement store can be bad. I believe it's only because manufacturers put a coating on the seed that could be bad. I'm sure others with more experience will chime in. There are many online sites that sell animal grazing mixes suitable for tortoise consumption for decent prices.
Those two are off the top of my head that sell seed mixes specifically for tortoises. There are other sites that sell pasture mixes for cattle that probably work well, too. Those sites might even be cheaper, but I don't know. Maybe other people can steer you in a better direction. All I know is that Tyler and Sarah at Tortoise Supply are great people and I've ordered from both links that I gave you.
This website sells a seed mix, the small $5 pack had plenty for a baby or they have a larger pack, plus FREE Shipping... Just bought some recently and planted them today
What is the coating in this case? A lot of them are bitter to avoid consumption of the seeds by birds and some other things. Usually does not apply to established grass.
I wouldn't use that potting mix necause it has a fertilizer in it. The package says "continuious feed for 6 months". The seed is probably okay because, like Baoh said, the coating is for protecting the seed and really won't be present in the established plant. However, there are much better seed mixes available. The sites Alma (pfara) have good grass mixes that give a variety of grass to the young tortoise. Also you can check out Peaceful Valley Farms for organic grass seeds. I grow the premium pasture mix and my tortoise eats it daily. http://www.groworganic.com/seeds/pasture-seed.html
seed from the store have stuff put on it so it has a better chance to sprout and to keep bugs off there are good sites like sulcatafood.com or carolinapetsupply.com desertseedstore.com and last off is arizonztortoisecompound.com they all sell a mix for sulcata and diff grasses that don't have anything on them that's the better way to go I think I use just plain topsoil with nothing in it