I have this guy in two wooden planters on my deck. Both are about the same size and I've had them outdoors year round for about 3 years. I have no idea what they are but in one planter it seems as there's an indent in the middle of the group where you can see down to soil and it appears as though the entire group is working it's way out of the soil. Could it be I just need to add more soil to cover the bottom/roots that are exposed? Or is something else going on? Overall the plant still looks healthy.

Good morning Lizzy. Love that sedum, and yes, you may need to repot the plant. It might have used up the soil. Or you could just add soil in the low spots. But I think since it is in a container I would repot with fresh potting mix.
It looks like Sedum SunSparkler® Lime Zinger, a wonderful sedum by Chris Hansen.

Thanks Lynn. I think that it's just used it up. I think that would tend to explain the "hole in the middle" of the plant. All the leaves are beginning to hang down over the part where it's raised so that it's no longer a tidy bunch. Thanks for the advice. Going to remedy the situation now!

Much happier
Before:





After:





No more discernible hole in the middle! I swear it looked like it had been laid on or something. Much better now.

It won't take any time at all to bounce back. Nice work Lizzy.

I noticed a similar effect with some Sedum Reflexum "Blue Spruce" I have been growing for years in wire hanging baskets, with the coco fiber liners. Between the used up soil and birds extricating the fibers for nesting materials, there were few fresh growing sections and lots of bare naked stolons. They were still wintering over well but badly in need of some freshening up. I took some cuttings and rooted them up for other projects as well as taking the remains of the baskets and adding them to provide ground cover in another two beds. They are recovering nicely.

Nice job on repotting yours. I'm sure they are going to love all that fresh new soil.

Thanks big T. That was my problem for sure. They were a little droopy for that afternoon, but have perked right up and look good as new! Previously, they looked exactly like you said yours did, bare stolons mostly in the center. They are happy as clams now and looking much better!

Way to go Lizzy. Glad to hear they are happy again.

Thanks Lynn. I'm hoping that they continue to do as well as they have been!