I had this happen to me few years ago. Removed the prop and found i've picked up a few meters of braid and failed the output shaft seals.  Eazy fix and not too expensive.  I had to flush the leg a few times with new oil to clean it out.

 If its a 2 stroke and you have just been running on muffs I would guess this as just unburnt 2 stroke oil

My Johnson does this quite often until it gets warmed up.

If its a 4 stroke then you may have a prop seal issue

Is it a tohatsu 70ish by any chance ?

   It's a 2 stroke 90 hp evinrude and yes it has only been flushed with muffs I've only had it in the water once  about 3 weeks ago, love only had it for a month now so hopefully it's only unborn oil fingers crossed 

I'm no mechanic, but that oil looks too black to be gearbox oil. Gearbox oil is a honey colour. That looks like unburnt 2-stroke oil to me.

Pull the prop off and have a look.

 It defenatly looks like unburnt oil to me seen it a bit on my old 2 banger and a few mates 2strokes

Cheers everyone

yeah I checked my gearbox oil and that is fine so hopefully only unburnt oil I didn't find any fishing line or anything else when I took off prop