As everyone else said it's a bit hard to see exactly whats going on from the photo's, but i don't think it would be a fungal infection, also ive never had an axi laying spermatophores in mid air (water) like that, normally they do it on a rock or somthin flat on th bottom, you've got some good rocks there, im sure if he was getting a bit frisky then he would use them. Probably just some fecal matter. I should mention though, that if you want them to bred and he is ready (females too) you should put some sort of aquatic plants in there so she can lay her eggs on/in them. Plastic plants are fine to use.