Behead and scrub! All seaponies get their heads taken off and a full cleaning when they come into my hands - 99/100, they're filthy inside. Toothbrush and pipecleaners are helpful for reaching the hard to get to areas, hot water, soap takes care of the rest. Never had to oxyclean 'em, be careful of the neon hair colors as they're prone to bleeding. Make sure they're fully dry before reassembly, same as any other.
Oxy won't take out the brown stains, sadly. It's plastic breakdown and/or staining from the glue. Do remove and discard any glue you find in adult sea ponies, there's usually a big lump of it holding down the weight and often drips on the inside. It's nasty stuff, and half the reason the adults show up with brown blotches so often. I also tend to leave out the weights if they're rusty, stored separately from the pony.