How to fix sour taste from Cream of Tartar?

A pinch of salt or a few drops of vinegar (white, usually) will achieve the same results as cream of tartar, also know as tartaric acid. CoT is an acid added (have you ever removed the cork from a bottle of wine to see a few crystals clinging to it? That's tartaric acid, or cream of tartar) to egg whites to encourage their strong protein molecules (ovalbumen) to relax and whip up to a gorgeous softness. Depending upon the quantity of your egg whites, I'm thinking that about 1/8 teaspoon of either salt or white vinegar will be just right. After the fact, however, the best you can hope for is to neutralize the excess acid with a bit of baking soda, which will likely compromise the texture, while still making your dessert more palatable.

Also remember that while cream of tartar is very helpful, its not necessary when whipping egg whites.

As long as the texture of your tapioca pudding will stand up to a little stirring, go ahead and neutralize the cream of tartar with a little baking soda. The flavor of baking soda comes mostly from its alkalinity, so it will be directly neutralized by reaction with the cream of tartar. Just go very, very easy with the baking soda -- start with a little pinch and see how that tastes after you stir it in.

Baking soda and cream of tartar are often used together for the combination of acidic and basic pHs. It sounds like it is too late to add any soda which doesn't seem like a good flavor profile anyway. How about sliced bananas or whipped cream to tone it down?