You probably already have found the answer to the question "Who attacked the Steamer Clara Bell", but if not here is what I have.

The Evansville Daily Journal: "About 25 miles above Grand Ecore, April 15, on the left bank of the Red River the rebels opened on the transports Clara Bell and Rob Roy, and fired twelve shots into them. A force of a thousand mounted infantry said to be a part of the rebel General Green's command came down in front of the batter and poured a volley of Musketry into the steamers which were getting the worst of it, when a small gunboat came up and silence the batteries, and compelled the rebels to fall back.
Gen. A. J. Smith's 16th Corps subsequently arrived near the scene of the action, having crossed over from Grand Encore, marched up the right bank of the river and again crossed over to push and punch the enemy. They captured 2 guns and 17 wounded prisoners who assert positively that the rebel Gen. Green was killed by a shell from the Rob Roy. This is another signal victory.
The rebels left 164 dead on the field, slaughtered by the terrific fire of our gunboats and transports, in their precipitated retreat. Their slightly wounded were taken off. By the death of Green the enemy have lost one of their most valuable officers. "

The Pittsburg Daily: "The Bulletin contains further particulars of the fight near Helena. Our total loss was sixty-two, including four officers killed. The rebels loss was 250. The Bulletin account of the destruction of the Clara Bell says: Four companies of the 6th Michigan Infantry were aboard at the time of the attack. Finding the boat sinking, the crew were about to surrender, when the officers of the regiment ordered the boat run to the Mississippi shore. The soldiers then fired her, to prevent her falling into the hands of the rebels."

Baltimore Sun: "25 miles above Grand Ecore, from the left bank of Red river, the rebels opened on the transports Clara Bell and Rob Roy, firing 12 shots into them. A force of 1,000 mounted infantry came down in front of the battery and poured volleys of musketry into the steamers, which were getting the worse of it, when a small
boat opportunately came up and shelled and silenced the battery, and compelled the rebels to fall back.
General A. J. Smith's corps subsequently arrived near the scene of action, and captured 2 guns and 17 wounded prisoners, who say positively that the rebel General was killed by a shell from our boat. (Rob Roy) The rebels left 160 of their dead on the field, slaughtered by the terrific fire of our gunboats and transports, in their hurried retreat."