I think you might be mixing terms. The SMA is the indicator and "GREATER THAN" would be the trigger. So if you're referring to drag and drop system design, you would DnD an SMA indicator on to the design surface and set the input to 20. Next, you would drag a comparison trigger onto the surface and compare the SMA to the closing price.
Is there a indicator which I can use to trigger a buy action when today's close price is GREATER than sma(20) ?