Based on the info given here without pics........
Algae is green !
Mold , mildew is black !
Pressure treatment can and will "lock" in the ripening of the wood as well.
Seen it a million times.
So as previously stated, it may not be the algae you are trying to attack.
Both look similar.
Is there a clear sealer on the wood?????????
Could be that your chem selection was too weak to break through the sealer ( if that's the case ).
Also if the wood was not acclimated for a few months and sealed immediately, that result could happen as well.
Using more pressure is not the solution, proper chems are.
Strengthen your mix and do a test area to gauge the mix.