Hot tub developed rough scaly surface.?

We just filled our hot tub for the winter after draining it last summer - summers here are too warm. We keep it covered year round, but we always thoroughly clean it prior to refilling it However, when we got in, the surfaces felt very rough and scaly. Very unpleasant. We are very diligent about adding chemicals and testing the water, but we have never had this problem before. Anyone know what this is and how it developed.