Are the tomatoes doing okay? Most fungi are actually beneficial and not pathogenic so if it's not causing any problems I wouldn't be to concerned.
Fungal spores travel through the air quite readily so that could explain the colonization. And yes, you may be overwatering.
Stick you finger in the soil and if it feels moist you don't need to water. another method to use is sticking a toothpick or chopstick in the soil and if the soil sticks to the wood then you don't need to water.