In Beowulf, what does Beowulf do when he hears about Grendel? Why is the Danish guard concerned about Beowulf's arrival?

When Beowulf hears that Grendel has been terrorizing Heorot, he decides to travel there to lend his aid to Hrothgar. This explanation begins on line 194. Beowulf is said to be beloved by the Geats, and they don't begrudge him his desire to help someone not of their own people. 
The Scylding guard sees Beowulf arrive on the beach with the fifteen men that he has brought with him, and all of them are wearing armor and carrying shields and weapons. The guard goes to them and demands of them their names and purpose; after all, Beowulf and his men have not been announced, nor do they have any kind of password or sign to convey their peaceful intentions toward the Scyldings, and so the guard has to assume they have arrived for the purpose of raiding. However, he immediately notices Beowulf's noble appearance, contrary to what one would expect of a raider, which may explain why he gives Beowulf the opportunity to explain himself.