Robbie is playing Ronnie Raymond, aka Firestorm, on The Flash series.
Amell teased, "Firestorm on the Arrow set. But why?"
If we are to assume Robbie Amell will be appearing in an episode of Arrow, it's possible that following the mid-season finale of The Flash, after Firestorm saved The Flash, he may have flown to Starling city.
Stephen Amell is wearing his longer hair, which suggests he is filming flashback scenes. I don't think Firestorm's powers include time travel, so Stephen looks to be filming scenes set in the past, and if Robbie is indeed filming Arrow scenes, his visit will be set in the present.
"The Flash" returns Tuesday, January 20th; "Arrow" returns Wednesday, January 21st.