Your layer contains a simple fill symbol that uses a nonsolid fill style. This symbol type cannot be set in the ArcMap user interface because it exhibits different behavior at different resolutions. It is supported in basemap layers and map services, but you should note that the symbol may draw differently than in ArcMap.
Solution
Do nothing. Simply accept the new symbol drawing as is.
Alternatively, you can change the simple fill symbol to a line fill symbol with a similar line pattern. For some styles, two layers of line fill symbols may be required. For more information on fill symbols, see Creating fill symbols.
More information
This warning message is telling you that the simple fill symbol may appear differently in a basemap layer or map service. This is because the fill of this type of symbol will not respect the reference scale and will always draw as 1 point in ArcMap's display. If your document does not have a reference scale and always draws at 96 dpi, you may not notice the difference in appearance. In general, line fill symbols are recommended because they offer more control and a better, more consistent appearance than simple fill symbols.