That is just some retained shed. Just give him a
bath once a week and gently scrub it with a soft toothbrush and it should come off eventually. Or it could just be a few missing scales which will grow back/come back after his next shed.
But I noticed that he is being cohabited with another beardie. You really should separate them ASAP because beardies are HIGHLY territorial animals and it is most likely causing them lots of stress.
Cohabiting beardies can cause:
-Stress,
-Health problems,
-Fighting,
-Dominance disputes which can lead to the less dominant one not eating and some other health problems,
-injuries (which can be severe and they could kill eachother),
-One or both not eating,
-The more dominant one getting the best Heat and UVB rays and the more submissive one getting the less good ones and eventually becoming more and more sickly.
There's about a million more reasons on why not to keep beardies together in the same cage but I pointed out the biggest ones.
I Would recommend putting the more dominant one inside a 120L bin/tub/tote with lights and UVB to house the more dominant one until you can get another tank for him/her.
-HippieLizards.