Usually when tubers sprouts and then stops growing, the tuber rotted. Sometimes they will begin to grow again if you pull it out and remove the rotten tuber and carefully plant what is left.

Well because I have things sort of densely planted removing it wold be a chore and I would probably break apart the tubers. Should I still do it?
OR what I'm thinking about doing is measuring it exactly now and verifying that indeed it has stopped growing before I cut anything. Maybe it just slowed down, but strange. I sure hope it is not rot.

If it is not rot, removing it for inspection will not kill it but only set it back by a week or so. If it is rot, it will look like the plant is somewhat wilted and not growing and this means it has no roots as they were on the rotten tubers. As you say, doing nothing for week or so will provide you with more information.

Thanks Ted,

I will definitely watch them closely and then decide if I really need to dig it up or not.
Z

Are they first year tubers?

In my answer (just now) to Califox, I noted that often my 1st year tubers in pots sprout and then seem reluctant to grow anymore. While I do not pack my pots with LOTS of other plants, I will share a larger pot with Glads or pansies for surface/annual color and it could be your other plants are getting the nutrients from the soil and leaving the dahlia short shrifted.

I think they will be fine and waiting is the right approach. But if it did rot, this is why many of us dahlia enthusiasts with LOTS of dahlias have 'graveyards' or ICU areas where we rehab poor growers or extra tubers just in casing we need to plug one in elsewhere.

They are first year tubers. I will wait and see. I guess it could be rot but everything around it including other dahlias didn't rot so hopefully it is just going slow.