What is wrong with my betta?

hello i have hada betta for about 4 months and suddenly this morning he was laying inside his cave which is rare because he never goes in there he usually very active but when hes in his cave and you tap on the gl he just stays in there frozen solid and occasionally comes up for air or when i feed him in the after noon and i have noticed a bi on his head and idk if it is ick or what is wrong with him i also bought him a "Tetra Whisper Tank Filter" and got him a 2 gallon hex aquariam and moved him from his previous 1 gallon square tank and abou two weeks in his tank he has was fine till this morining does anybody know whats wrong with him?