Adam, I'm just a fairly new pond owner, not an expert. But I can say that if it is blue green algae, please don't allow your pets to swim in it or drink it. Can be toxic. I wouldn't eat any fish from it, either, at least until the algae is gone.

Also, Bob Lusk has drilled this into our heads: The real problem isn't the algae. Rather, algae is a symptom of the real problem, which is excess fertility (likely phosphorus).

If it gets worse after a rain, most likely runoff from highly fertile soil. Reduce that source, and the algae won't be a future issue. Kill it but don't change the runoff, and it will be a future issue.

7ac 2015 CNBG RES FHM 2016 TP FLMB 2017 NLMB GSH L 2018 TP & 70 HSB PK 2019 TP RBT 2020 TFS TP 25 HSB 250 F1,L,RBT -206 2021 TFS TP GSH L,-312 2022 GSH TP CR TFS RBT -234, 2023 BG TP TFS NLMB, -160