Congratulate on your new addition! I haven’t kept African blue stripe pipefish before so I can’t say for certain whether or not your pipefish might be inclined to dine on newborn seahorses once it is fully grown.
However, I can tell you that I find dwarf Gulf pipefish to be wonderful tankmates for Pixies or dwarf seahorses (Hippocampus zosterae). I’ve had adult breeding pairs of the dwarf Gulf pipefish that were 3-4 inches in length and they never showed the slightest interest in the tiny newborn Pixies. Their mouths were large enough to swallow volcano shrimp or red feeder shrimp (Halocaridina rubra) but they never bothered my dwarf seahorses or their babies in any way. So if you’re blue stripe pipefish is anything like the dwarf Gulf pipefish I’ve had, and it doesn’t grow any bigger than 3-4 inches, I suspect it would not be a serious threat to your seahorse fry.
The larger species of pipefish can and will predate seahorse fry under certain circumstances, but your little blue stripe might be okay, although there are never any guarantees about that sort of thing…
Best of luck with your pipefish and seahorses, Nigel!