In-gamewise, there's no indicator of what the Secret Triumph is. There is a Secret Triumph, although both the objective and reward are hidden. However, using a tool like Braytech or Destiny Tracker shows what appears to be the actual triumph:
Now according to this, the secret triumph requires me to see a spider in my sights. I don't believe this pertains to the NPC Spider of the Tangled Shore considering a few things -- one, your weapons are disabled when you are in line of sight of Spider, so you can't scope in on him or even aim your gun at him. I remember back in Destiny 1 where as an easter egg, aiming down sights had a chance for a spider silhouette to crawl across your screen momentarily. I've played around with scoping in for large amounts of time and have not seen anything.
In any case, how do I complete this secret triumph for the Festival of the Lost event of 2019?