Sun will help, you can leave it in the sun taking care the whole affected area gets direct sun then, still outside, brush off all that is possible with a stiff brush. After that you can proceed to a proper wash.
For the wash you can use oxygen bleach, it will have a better outcome than regular bleach.
From there on it really depends on the conditions of the pack after the wash, you might have to repeat the process.
Keep in mind that even if the mould is gone there is a good chance it will smell like mildew, that is a difficult smell to remove sometimes and that some discoloration can remain in the area affected.
Remember that the brush will have to be treated as the pack is after you used it on the mould.
Edit to address the residual mildew smell, which is likely to remain if the mould was a lot:
- in conservation, commercial products that will bind the leftover chemicals causing the odour are used
- I tried Mirazine in the past, always gave poor results for mildew.
- there are home remedies, vinegar being the most common
- on boats, beside the usual commercial stuff, its often used borax or glycol (plain antifreeze works too even if its propylene glycol) This stuff needs proper disposal after being used as its a wash, the intent in this case is to remove the smell not to use these substances as biocide, the process for that is slightly different and not applicable to the question.
- YMMV depending on conditions and material treated.
- Do not use Feebreze or similar to cover the smell or you will end with a carousel of stenches that will drive you nuts.