I don't think there is anything to give as a preventitive if it's drainage from bites. My barn did decobweb when we started seeing these and that seems to have done the trick.
I had two that showed stuff like this and they were both....grey. One was probably spider bites. The other a reaction to an overdose of routine meds and supps. 5 baggies with day of the week right on them, dumped all at once. By an idiot BO who returned from a 3 year absence and immediately instituted "minimalist" management theories. Cost me $600 in vet bills.
Lumps went away in 10 days. That was 9 days after I left that barn.
When opportunity knocks it's wearing overalls and looks like work.
Please keep us posted. My guy had so many yesterday (and they were huge- all shoulders and back) I could not put a saddle on him. I would love to cold hose him but not when it is 50 degrees! The anti hyst seems to be not helping. Ps I am also in Indiana???? Weird.