Stick your finger in the potting compost, deep and feel how moist or dry the thing is: dahlias don't like having "wet feet" and are extremely prone to root rot, which on pretty much all plants starts manifesting with yellowing, drying and finally dying leaves. Dahlias take very little water, even as potted plants, and if root rot has started there's little that can be done to save the plant.
Now another thing are those tiny white spots on the leaves. They kinda look like the leftovers from a pest feasting, perhaps whiteflies or lacebugs. Any chance you can try wipe them off with a paper towel to see if they come off and how they feel like?
