Muck in my tank

[sanben1964]

Hi guys

There is some kinda muck which settled on the plants I'm not sure what it is,
Advice pls how to get rid of it, water in the tank remains to be clear.

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Thank you

 

[Delapool]

Not detritus or fish poop or fish food out of the water?

 

[sanben1964]

I'm absolutely unable to identify it

 

[Delapool]

Does it rub off?

 

[sanben1964]

Yes it's some kinda stuff not sure from where it's coming; I've got a good external filter too