Dectomax is unlikely to do them any harm, beyond the usual side effects listed on the data sheet which can include things like lump at the injection site, possible reaction to the drug. Any in lamb could get stressed by catching to inject and can cause abortions.
If it is the wrong drug for the type of creature causing the problem, then it is a waste of time and money to use it, and would contribute to wormer resistance by using a wormer/parasiticide that is not required.
From the photos it does look like a parasite of some sort, but i cannot say if it is lice/mites or what type of those. Some will be killed by dectomax but some will not, so it would be best all round to check what it is you are dealing with.
Hope you get to the bottom of it soon