Observations by dompf

Some kind of microbial growth, presumably algae...blue-green spots on surface of water

It grew over the course of the summer, in water containing acidic toilet bowl cleaner (see label). The intention of placing these packets inside the toilets was to prevent microbial growth, which is typically the result (see image demonstrating what water treated with the same chemical typically looks like, simply pink water). Additionally, the packets of pink acidic toilet bowl cleaner do not stain the toilet bowls, even when left over the summer. The toilet with this growth had a muted-purple colored stain (see image), despite the fact that the packets are pink, and the growth was green. This stain did not come up easily, not even with a stiff bristled brush and cream cleanse, a gritty toilet cleaner that contains bleach.

Observed on a picnic table, under a tree. It probably dropped out of the tree, and if I remember correctly from my childhood days of tree climbing, these things have a moderately painful 'sting' from what I believe is used to bore into the bark (mandibles grab hold and abdomin is thrusted downward). I don't quite know if it's an insect because I'm not sure if it has 6 or 8 legs. Very aggressive. Roughly 3mm long, "head to tail."