Lawnmower Blenny and Tang fighting

This is really strange but I've had my blenny in the tank for about 3 months now, and the Yellow Tang came in about 2 weeks later and all was good. The Tang would go right up to the blenny and neither of them had any problems with each other. So just a few days ago I added a small emerald crab to the tank and it seemed like it set off the blenny and he bit my tang 3 times (i counted over 2 days) and left a nasty cut on the side of the tang. I can't see why the emerald crab would have caused the blenny to attack my tang but has anyone else ever heard of a Blenny getting aggressive to other tank mates later down the road? The emerald crab isn't even in the same area of the blenny but its the only thing i could think of that has changed. I'm in the process of helping the tang recover (selcon solution additive to food, more seaweed sheets w/ garlic, increased water changes, and stress coat) and she is looking better but I'm afraid the blenny may attack the Tang again. What really ticks me off is my Tang is finally recovering from lateral line disease then this happens! Any suggestions would help, though I'd like to avoid removing anyone if possible.

 

smack rock

I had an identical situation with an algae blenny and a Hippo, there was no problem when the Hippo was 1st introduced , but some time later the blenny, decided it hated the tang, and went looking for it to head butt it (not just passing aggression- it hunted the tank for it)

I believe although Im not sure , that it was a territoriality issue based on grazing rights
maybe one day the Hippo grazed from a rock the blenny consider its own, and it never forgave it there after

I had no choice in the end but to seperate the 2 fish as it never ceased, just on some days it was more intense than other days (possibly a re visit to the same rock by the hippo)

Steve

 

Lawnmower blennies are regarded as peaceful fish but since I had mine, it killed my mandarin goby and only this morning, I noticed that it is constantly attacking my magenta dottyback:-/

 

Rough, doesn't sound too reassuring. Well I'm going to monitor the situation, looks like no bites today but like you said steve could be on and off. Thanks for the inputs.