As @greenhorn suggested, the only thing I'm willing to try is run it on a cheap player till it drops the powder. Since I can't find my old walkman, that's ruled out.In my personal opinion once your tape suffers a fungus attack or starts dropping oxide, it’s time to just discard it. If the recording matters, try a cleaning exercise and record out the content. Playing these will lessen the life of the pinch rollers and heads (constant cleaning). The only way to avoid this is to follow the cliche - prevention is better than cure
I have 10 thumbs and any tape subjected to the ignominy of being opened up by me for cleaning has its fate sealed!
Looks like I will have to discard them.
It's a relief though to know this is not fungus but chemical degradation. Can't spread to the other tapes like fungus, can it?