Most pobably bleaching... yes they will recover from proper care.... whats species is your clam if its maxima or crocea.. place them high up on the rocks.. especially so if you have only 150watt
Clams dun turn brown.. instead they bleach or their colour becomes shabby...... yes your lighting is too low especially when the height is 2.5feet...... oh ya they dun STN too(they waste slowly more probably)....
i dun think you got sting by its finnage. probably got sting by its scapel or sharp spine(wadeva you call it) that is potruding from its gills... anyway this is a defence machanism for angel and you shoudl be alright after soemtime IMO