red stuff in soil

[ghftde24]

So I went to repot one of my cacti that had fallen off of a ledge and when I went to mix up the soil( it seemed like it needed a little more sand in it) I noticed a bunch of red areas in the soil. At first I thought it was just some of the lava rock that I used to put on top of the soil but when I tried to pull it out of the soil it was the same consistency as the soil and just crumbled. It is the same exact color as the lave rock that I used but I've never heard of lava rock dissolving before, as far as I new it would stay solid even if it was wet for extended periods of time. I'm wondering if it could serratia marcescens because from the pictures I've seen it is the same color as the stuff that I found in the soil, has anybody else ever had this kind of thing happen with one of their plants before? I put mycorrhizae into the soil but I've never seen this on any other plant that I've grown until today. These were the best pictures that I could get of it the second two show that color the best but the first one has the best detail, for some reason my phone takes blurry pictures when the flash isn't on.

[applestar]

I think it's more likely "rust" fungus on the cut end of the cactus/succulent.
Not sure how it should be treated, but definitely more sand 50/50 -- especially immediately around the plant. 1/3 sand in general.