I don't recommend any "in tank" cleaners. These seem to ruin the rubber and plastic parts. Not sure it is mold (although it could be) but you could pour bleach or 50/50 mixture of bleach and water directly into the hole located under the flapper (after turning off the supply valve and flushing). This is the route the water takes when the toilet is flushed. Let it sit for about 10 minutes and then flush. It will kill any mold. I use vinegar a couple times a year to help with calcium deposits which also form in and around these holes. Using a mirror, I check to make sure none of the holes are completely clogged and if so, a wire coat hanger can help clear them.