0.2ppm is actually not that high but high enough really. Any amount of PO4
will help fuel algae growth. I cannot comment on the accuracy of the Seatest kit as I have never used that brand. If the measurement is true or even lower, then the best thing now is to ensure that no more PO4
is introduced. The remaining amount will eventually be burned up by the algaes.
Top off/change water and foods (mainly frozen) will be your greatest concern for additions of phosphates. Now that you have the test kit, I would suggest ensuring that these sources are not contributing.