A sustainable competitive advantage is a long-term strategy or process that allows a business to remain ahead of its competitors. Unlike short-term advantages.
In my opinion, competitive advantage considers operational and financial excellence in business to overcome competitors whereas sustainable competitive advantage considers financial, environmental and social excellence in business to compete rival firms. At the same time, as @Reza Lalehzari mentioned SCA is a long term strategy and requires overall improvement in business.
Both confer the same meaning, but with a very big difference. Competitive advantage could be associated with immediate edge, but, could probably be short term, temporal scale, with huge benefits, accompanying it. Sustainable competitive advantage on the other hand would, is still an advantage, coming out slowly probably, but, spanning to eternity. We hardly shed tears, if, we were to be lucky to achieve the later.