Those were probably fungus gnats. When you see fungus gnats its usually means that your soil is to water logged or staying wet for a long period of time. Jades and other succulents hate to be wet for long periods of time. Lightly squeeze the trunk at the base to see if its squishy. If it is, the best bet you would have is to cut the tree off wherever the trunk stops being squishy, dry it out for 3 - 4 days then plant it as a cutting. If its not squishy then put a fan on it so the soil dries out as fast as possible, and I wouldnt water it again for a week or two. Jades could probably go months without water so 1 or 2 weeks wont hurt it at all. And for future reference I would only water this after the soil is literally bone dry for a day or two and then give it a good deep watering. Also these plants do way better outdoors if possible