That is prob hard water deposits, gets it whenever I splash water on the tank & letting it dry. Using a vinegar soaked rag will soften the deposits & make it easier to remove.
One thing to check is that it is not the start of a slow leak. Sometimes water that is slowly leaking out will dry before it puddles, forming these deposits. <After you remove the stuff, dry & check that it doesn't get wet all on its own.>