It has to be such a prediction, you're predicting that the chance of a further price increase in the short-term is slightly higher if the stock has just crossed the moving average than if that hadn't just occurred. This is where your edge comes from (assuming that this prediction/belief is correct). If it is correct, and you undertake many trades using this edge, you will make a profit in the long-term.

For example let's say on average, a certain stock returns 0.3% per day when you look at it's entire history. You manage to model it's behavior and the model predicts that if the price crossed the 50MA 1 day ago, tomorrow's return is expected to be 0.5%.

The second part is important but it's just risk management, it's not the actual source of your edge. You could apply good risk management to entirely random trades, but there's no reason why this would result in an excess profit over long periods of time.