If you want to use something organic, you can use EVCO. It worked like a charm on a DT I recently ceased and adopted.Almost certainly mites. I've not seen them congregate on tortoises before but a little mild soapy water sprayed over them will almost certainly rid the tort of the 'infestation'. You can also use diluted flea/tick shampoo made for cats & dogs - 1 tbsp per gallon should do the trick. You can also spray that in the enclosure to eliminate recurrences. I've used this method to eliminate ticks to which certain species of torts were subject.