Other than a fog bank near the horizon, a number of other explanations might potentially account for the observed feature. However, none of these alternative explanations appear likely. One alternative is that the observed feature is a low-level cloud. This is unlikely because the shape and position of the feature did not appear to change much throughout the observation duration of over 20 minutes. For comparison, clouds are known to change shape and move on time spans of less than a minute. Nevertheless, the possibility that the observed feature is a low-level cloud cannot be ruled out entirely.
Another explanation is that the observed feature is a mirage. A superior mirage occurs when the image appears above the horizon because the ground is colder than the surrounding air, while an inferior mirage occurs when the image appears below the horizon because the ground is warmer than the surrounding air. If the observed feature is a mirage, it would be a superior mirage as it is above the horizon. However, as the Huygens probe descended though Titan's atmosphere, its measurements indicate increasing temperature with decreasing altitude, and that the air above ground is cooler than the ground itself. As a result, a mirage is an unlikely explanation for the observed feature.
Smith et al. (2016), "Possible ground fog detection from SLI imagery of Titan", arXiv:1603.04413 [astro-ph.EP]