erc-command-indicator

erc-command-indicator is a variable defined in `erc.el'.
Its value is

nil

Documentation:
Indicator used by ERC for showing commands.

If non-nil, this will be used in the ERC buffer to indicate
commands (i.e., input starting with a '/').

If nil, the prompt will be constructed from the variable `erc-prompt'.

You can customize this variable.