You could use a conditional format in Excel like this:
import pandas as pd
# Create a Pandas dataframe from some data.
df = pd.DataFrame({'Data': ['foo', 'a --> b', 'bar']})
# Create a Pandas Excel writer using XlsxWriter as the engine.
writer = pd.ExcelWriter('pandas_conditional.xlsx', engine='xlsxwriter')
# Convert the dataframe to an XlsxWriter Excel object.
df.to_excel(writer, sheet_name='Sheet1')
# Get the xlsxwriter workbook and worksheet objects.
workbook = writer.book
worksheet = writer.sheets['Sheet1']
# Add a format. Light red fill with dark red text.
format1 = workbook.add_format({'bg_color': '#FFC7CE',
'font_color': '#9C0006'})
# Apply a conditional format to the cell range.
worksheet.conditional_format(1, 1, len(df), 1,
{'type': 'text',
'criteria': 'containing',
'value': '-->',
'format': format1})
# Close the Pandas Excel writer and output the Excel file.
writer.save()
Output:

See Adding Conditional Formatting to Dataframe output in the XlsxWriter docs.