USD amount in your wallet can change depends on market price of Bitcoin towards USD. It can go up or down. We are refreshing current BTC price every 5 minutes so even if you are not mining at the moment but you have some funds on your account you can see the difference in your total funds USD amount. But amount of Bitcoins in your wallet does not change. whichever number you mined till this moment that will be your number and will not drop you can be sure about that.