When you unrack and get in the position to squat you are 'dancing' with the bar. You have posterior pelvic tilt in the bottom of the squat (your lower back rounds). You lose tension for depth. Could be inflexible hamstrings/glutes and/or weak lower back (for this weight) or just a technical issue.
Try shifting you upper body a little bit more forward while pushing your knees out more on the descent, see if this helps.
Focus on fixing this 'problem' before you put more weight on the bar, when you get it right the weight you have on the bar will explode up.
Just finished watching the three videos I found on here. A LOT of helpful tips. (I'm zooming through the forum on any squat related posts for more info). I know I need to tighten my core and back, that's why I started with the lighter weight. I did go up to 315 but, with high box squat.
I think the 'dancing' is just getting the manta ray "settled" a little better. I'll check it again next time. Watching the "so you think you can squat showed me that big time. I'll definitely work on it.
Shifting forward, meaning I need to lean forward a little more? Not so erect?
That's why I posted here, I need to fix things before I hurt myself with more weight.
Ok I didn't see the mantaray, sorry for that. I would say stop using it and only use for 'confusion' training When Your normal squat is good, because I guess you want to squat low bar. What i mean by Shifting forward is that it should begin from the start of the descent and be kind of constant When you lower. I think point 5 and the pictures in this article show What I mean davedraper.com/blog/2009/09/16/deadlift-stud-squatting-dud/
Well, I use the manta ray so I can stand more erect rather than stooping over to off set the bar. With only one-arm, I can't (don't feel comfortable) squatting more than 135lbs. Plus, I'm not sure it's safe with the amount I need to lean forward. So I use the manta ray to help keep the bar on my back and I can increase the load.
In addition to the back tightness, it really looks like you have some abductor flexibility issues. Trying to do the following Cossack squat progression (with obvious adjustments). They were very helpful for me.
Also, seeing as you can only do high bar squats, I think a pair of weightlifting shoes would be a wise investment.
Sorry dude, I really didn't see that you only have one arm, shame on me. The mantaray makes your squat pretty high bar and this is the best option for you as you tell. With the high bar squat your body should indeed stay more erect and the info I gave earlier doesn't apply, except for the flexibility things. Work on hamstring & glute flexibility while getting your quads and lower back stronger.