Our contractor also just installed an acrylic tub for us and we have durock on the walls surrounding it. I can't really understand the context of the picture you provided. I am not a builder, but having just seen ours installed, I pretty much know what the contractor did.
We have a concrete slab floor and the legs on the new tub were adjustable. The builder and his helper went thru great pains to make sure the tub was level. The manual said not to put concrete under the tub except maybe some around the legs. It was optional and we didn't do that. Then he glued the centre of the legs, where they adjust to sort of lock them in place so they won't change over time. Likely not necessary but just a precaution. He used 1 x 4 or a 1 x 2 (not sure) strapping behind the tub as per instructions just for the tub to sort of rest on. Then with the tub full of water, he used stainless steel screws to screw thru the holes in the flange along the edges of the tub into the studs in the wall. The durock was put on the wall just to the top of the flange.

Our finished solid surface (or tile) will be put on it once waterproofed with redgard and will hang over the flange but not touching the tub. That little space will have silicone caulking to seal it.
Ours doesn't have a removable apron or any flange on the side so I am not sure which part you are showing in your photo. What good would putting a screw above the flange do??
I think you should fire this guy and find someone else to carry on with the install.

Editing: I just Googled your tub style so I think I see what that piece of flange is. Since it is on the side and outside the rim of the tub, it is likely safe from much water. I would likely fit in a small piece of durock if you can and when it is all taped and waterproofed it should be good.
Hopefully some of the experts will weigh in but if there are holes in the flange, they need screws in them. Your manual should explain the proper way to install it.

The picture shows the horizontal flange and the vertical flange of the apron. My contractor said they tried their best to fit the durock around the tub edge and they cannot do it better. I tried to upload a larger picture of the tub with he hole right above it. My contractor didn't want to use red guard or any membrane, as much as I know he did't put anything behind the durock either. He didn't put the seam tape in mortar, and just put red guard over it which he wasn't going to use at all. next step was to tile the whole thing without waterproofing it. I am glad he walked out. I am just wondering if I need to redo the durock or fill it somehow. Can one take durock down and put it back or is it not usable anymore?

I just edited my comment while you were adding yours. I think it depends which kind of tape as to if you use mortar. I am doing my own waterproofing but the contractor said to get the mesh tape and tape all the seams. Then do two coats of red guard over the entire durock, get corners good etc. I hope if the tape needs mortar, someone will say so on here.

Are you saying he only red guarded the seams? I can see in the picture the durock is not redgarded. The entire sheet needs it.

He only put red guard over the mesh tape. the tape is sticking off the durock and is now embeded in the rubber not the durock causing bumps to stick off. I dont think i can tile over this mess. I just hope I can rip the red guard off without doing too much damage to the durock and retape. He wasn't even going to use any membrane and just tile over the durock and mesh and the tub moving around. I wish I hadn't paid him for the materials yet since I might to even rip out the floors as well.

It might be best to cut your losses and take all the Durock down and salvage what you can of it and buy more if need be. But nothing to lose by trying to pull the tape off if it will come. See what is left of the durock! Durock absorbs water and even with urethane grouts on tiles some moisture can get thru.

Hopefully some of the experts will chime in here. I will follow closely as I am a few days away from Redgarding my own. We did a small shower a couple years ago with Durock and Redgard but I just want to make sure I remember proper procedure.

Did you check the installation instructions for the tub? Ours is a different brand ( Neptune Wind) but I would think installation would be similar. So far ours seems real sturdy but I haven't had a chance to actually use it of course!

I did check the installation and it said it needs to be set in mortar if not level. also to rill pilot holes in flanges to screw into studs. though I was reading if flanges are not very large, which is the case here, it could put stress onto the acrylic over time and crack. Others suggest to put a ledge under the flange on all three sides to support the rim and prevent it from flexing too much. The front apron is very flimsy and the brace it comes with is not very supportive. I think I will need to do a lot of extra support to make this tub stable enough.

Neptune install instructions

Here is a link to our tub manual. I know it's a different brand, but it shows the ledge all around the tub for support. For mortar you just put the feet in it. Ours doesn't feel flimsy so maybe there is some difference in how it is made or just because the style is different. Ours had holes in the flange for the screws-unless our contractor made them and I didn't see him do it. Our Durock was put up before he screwed it to the studs. (just because he was real busy and had a helper putting up the Durock). After that the tub squeaked when I just lightly leaned on it and I lost a night of sleep worrying about it. But once he filled the tub and screwed it to the studs, it improved 100%. No more squeaking and very solid.

I bet there is a difference in quality of the tub. Mine was not very expensive compared to the average price of a new tub. I also think it feels flimsy because it is not installed properly. I have spent the last few hours reading on how people put in tubs and I think the ledger is a very good idea in this situation as well as a back brace behind my apron as it moves around a lot. the brace it came with is only supporting the center and not the sides. I hope my overkill on this will hold up. Or else I will need a new tub.