Rain Rot?
With all the rain some folks have seen the old nemisis rain rot is likely to show itself.
When I noticed the little clumps I have used both Listerine (or inexpensive mouthwash) and Absorbine liniment.
I never had any luck with any iodine based shampoos.
Make sure you bleach any tack/grooming items that come in contact with your horse for good measure.
For Uses for Absorbine please visit their website HERE