What you describe is called creep. According to this technical manual
Creep is a material property frequently misunderstood and can be defined as the continued extension of a material when subjected to constant, long-term static loading.
There are several types of dyneema and some of them have lower creep resistance. Balance community says
Some of the earlier versions of Dyneema suffer from severe creep issues. Creep is the process of plastic deformation under load. That means when you tension a piece of Dyneema, it will continue to loose tension and physically become longer over time. New versions of Dyneema have started to mitigate this issue, with the creep properties of SK-99 being substantially better than all of it's predecessors. This issue only becomes a problem when using the fiber near it's breaking strength or if left under substantial tensions for a long period of time.
Now, is that an issue? Should you retire your dyneema sling because of creep? Reading about creeping in applications more serious than slacklining (for example, in ropes for boat anchoring), a mooring company says:
Creep has two important effects in ropes. Loads will equalize between the different yarns, thus rope strength will increase in use. Secondly, it can be used to estimate the safe working life of a rope. In addition to permanent deformation or creep, a higher elongation will occur in a new rope while the fibres or rope adapt to the structure of the load.
A worked rope or pre-stretched rope will therefore show less elongation than a new one. As the rope is put under tension for the first time, the fibres rearrange to provide better load sharing, resulting in a gain in the breaking load of 10%.
Which means that, since you're not worried about the fact that the sling got longer, but if it got weaker, the answer is no - at least superficially. You can confirm your sling is still intact by looking at its fibre structure (taken from balance community's website):
This figure is obviously about ropes, but it also applies to slings. Check you sling fibre structure: if it looks as anything above a 4 in the picture, retire it.