Well, EURUSD, USDJPY and the cross EURJPY -they all depend on each other,or shud say "connected".
Sometimes big orders in EUR/JPY are filled thru major pairs (for best liquidity).
If EUR moves, then probably cross will move and vice versa.
If EURUSD goes up, somebody will wish to sell USDJPY and so on.
However, if you are open in EURJPY you may forget about EURUSD and stared just at the cross chart
If you are open in EURUSD, you may ignore EURJPY movements if you wish.
Triangular arbitrage is only in some "manuals". You waste your time looking for it.
You better try to catch arbitrage on EURUSD if you have accounts with two brokers.