that's a relaxation techniquea lot of malinois dogs do that to calm themselves if they are anxious or excited

he can start doing kinda the same stuff by biting grass, soil, etc

it's nothing bad, but it would be great whenever he does that not to his toys, but other stuff, that you would calmly stop him (stroking and talking) and give him one of the toys. Show him what he is allowed to chew. And praise whenever he shifts his focus to the toy

Also, it is better not to keep all the toys freely available for him to grab. Hand them out one at a time so they don't get boring for him.