I have a 10 gallon reef with 2 Scarlet Leg Hermit Crabs, 1 Margarita Snail, 1 O. Clownfish, 1 Yellwo Watchman Goby, 1 small Zoa frag, 1 Mushroom, and 1 frag with 5 heads of Candy Cane Coral. Also about 13-15 pounds of live rock. I added 10 pounds of live sand (Nature's Ocean Bio-Active Reef Sand) but I just recieved a Yellow Watchman Goby. Can I safely add more sand to my tank without causeing it to mess up or is it too late? Its been up and running since January 3rd I think but i'm not forsure. Thanks in advance!

You can add more.

I would use well rinsed dry sand.
I would then add is slowly a cup at a time lowering slowly and pouring slower to keep all the dust to a minimum. If you notice to much of a cloud stop and let it clear before continueing.

This is just how I would do it others ideas or approaches will differ.

One of the best ways I've seen to add sand without causing a sand storm is by funneling it down a pvc pipe.

I'd be careful adding it and monitor your permiters closely afterwards. have some extra water on hand mixed and ready in the event you need to do some water changes.

could I add more Live Sand? or will it cause a crash?

I've added sand to both of my tanks without issue. But , I got mature sand from my Lfs down the street. I would not recomend adding prebagged LS.

Dang lol I was wanting to add pre bagged live sand.