Sorry to hear your algae came back.

Borates (boric acid) are a mild algaecide, not a strong one like sufficiently high levels of chlorine or copper (but that can cause staining). It's possibly a little weaker than PolyQuat though that is just a guess. Mustard/yellow algae takes about twice as much chlorine to kill it as regular green algae and it probably takes a stronger algaecide to inhibit it.

A chlorine level that is 15% of the CYA level seems to be roughly the level that inhibits its growth and is roughly the "Max" column in Ben's Best Guess CYA Chart. Killing an existing outbreak can take an FC level that is 60% of the CYA level over several days and making sure that you get behind light niches, soak all poles, etc.

Richard