erc-mode-line-format

erc-mode-line-format is a variable defined in `erc.el'.
Its value is

"%S %a"

Documentation:
A string to be formatted and shown in the mode-line in `erc-mode'.

The string is formatted using `format-spec' and the result is set as the value
of `mode-line-buffer-identification'.

The following characters are replaced:
%a: String indicating away status or "" if you are not away
%l: The estimated lag time to the server
%m: The modes of the channel
%n: The current nick name
%N: The name of the network
%o: The topic of the channel
%p: The session port
%t: The name of the target (channel, nickname, or servername:port)
%s: In the server-buffer, this gets filled with the value of
`erc-server-announced-name', in a channel, the value of
(erc-default-target) also get concatenated.
%S: In the server-buffer, this gets filled with the value of
`erc-network', in a channel, the value of (erc-default-target)
also get concatenated.

You can customize this variable.