its mold or something feeding on the ant bait and it is probably harmless of itself. The ants will kill your plants if they are numerous, they disturb the roots and allow air pockets in the soil that will dry out the roots. One thing ants do not like but hostas do, is water. Go out every day and water liberally and the ants may move to another location. Also if you can find ants elsewhere in your yard or surrounding area and then take a shovel of those ants and toss them on your problem ants, they will kill eachother. I mix borax into pancake syrup and then pour it around the foundation of my house to keep ants out, it doesn't work 100% but it really helps.
