Use embedded content processing in the Excel file type.

Enter \[.+?\] as placeholder:

Then you simply use the XML parser and set up its embedded content parser like this:

Start tag: left square bracket, followed by one or more "not right bracket" characters, followed by right square bracket

\[[^\]]+]

End tag: left square bracket, followed by slash, followed by one or more "not right bracket" characters, followed by right square bracket

\[\/[^\]]+]

Leaving the default segmentation hint "May Exclude" should be okay, as long as you DON'T need to see the tags in editor.

Using the XML parser with legacy embedded content parser would be probably better in this case as this would result in "cleaner" content in editor, without gazzillion of "internal file separators".

This is the XML parser settings, here you configure "where the embedded content is stored inside the XML".

But, as I mentioned, you should have perhaps better used the "XML (Legacy Embedded Content)" in this case... There you can define the tags directly in its settings.

If you really want to experiment with the "new" XML parser, the process is a bit more complicated...
First you should create the plain text embedded content 'file type' - go to Embedded Content Processors, make a copy of the Plain Text type, give it some nice name and define the tags there.
Then go back to your XML parser settings, turn on the embedded content processing, select the plain text embedded content processor you just created from the dropdown, and then select "Element" from the dropdown on your screenshot - this will tell the parser to consider content of each parsed XML element to be "the one containing the embedded content" and "the one whose content will be sent to the embedded content processor for processing".
And you should be done...