That arrow is the multi-line indicator. That is especially useful if you are having quoting issues, which is a common problem for many novice users. It is not in the output of the template though, only the actual value is.
this does not change an icon, why are you still using the ‘on’/‘off’ there? did you search the MDI repository Patrick showed you, and noted the actual icon names ?
why not finally show us what you actually tried completely?
please help us help you… btw see The Home Assistant Cookbook - Index
let me help you:
change icon template
mdi:power-plug{{'-off' if is_state('sensor.tronity_q4_sportback_e_tron_charging','Disconnected')}}
will do it. explanation:
this set the mdi:power-plug by default, and only if the sensor is ‘off’ will add -off to that making it mdi:power-plug-off
give it a try (o, and check if the state is actually ‘Disconnected’ and not ‘disconnected’, or ‘off’ )