Here is your code tidied up a bit. Note that the toggling of field codes makes an assumption that codes were not showing before the macro ran. That is risky so I changed those to explicitly set the value instead of toggling true/false.
Code:
Sub ReplaceCaptionStyle()
' Developed by Andrew Lockton (Guessed)
' Edited by laith93
' www.msofficeforums.com
' Showing all field codes (or by pressing Alt-F9):
ActiveWindow.View.ShowFieldCodes = True
With Selection.Find
.ClearFormatting
.Replacement.ClearFormatting
' Replace "CapTbl" with your own alternative caption style for tables
.Replacement.Style = ActiveDocument.Styles("CapTbl")
.Text = "seq table"
.Replacement.Text = ""
.Forward = True
.Wrap = wdFindContinue
.Format = True
.MatchCase = False
.MatchWholeWord = False
.MatchKashida = False
.MatchDiacritics = False
.MatchAlefHamza = False
.MatchControl = False
.MatchWildcards = False
.MatchSoundsLike = False
.MatchAllWordForms = False
.Execute Replace:=wdReplaceAll
' Replace "CapFig" with your own alternative caption style for figures
.Replacement.ClearFormatting
.Replacement.Style = ActiveDocument.Styles("CapFig")
.Text = "seq figure"
.Execute Replace:=wdReplaceAll
End With
' Hiding all field codes (or by pressing Alt-F9):
ActiveWindow.View.ShowFieldCodes = False
End Sub