the best option speaking from my limited experience is to quarantine the little guy if possible then use one of the following two treatments (which according to most sources are the best). there is the hypo-salinity treatment which basicly is dropping the SG down progressively over say 2 weeks too about 1.012 - 1.010 then raising it progressively. personally i have never tried it but i have read many people have and its brilliant. second is treating with copper (but as you mentioned you cant due to invertes) there are the options also in regards to ich solutions however i have tried myself everything under the sun with no glory.
i would suggest monitoring it say over 3 days if possible in quarantine, if the powder is swimming around, eating and acting fairly normal with some white spot leave her/him alone! they probly will be just fine, if however they start scratching constantly.. like they simply lie around hardly no movement then they scratch! or if they start not eating and are not energetic need to pick an option and do it but unless they are literally covered in ich dont rush it and dont panic too much
